Who are the credible characters in Arthur Conan Doyle's Hound of the Baskervilles?

As with most crime fiction, most of the characters withhold information or lie about it. What creates and resolves suspense in this sort of story is precisely the way readers are only gradually given the information needed to fully understand the events we witness. For characters:
Narrator: The third person narrator is reliable but is mainly restricted to the viewpoint of Watson and only knows or reveals information to us gradually.
Watson: He has an open and honest character and tells the truth, but is not as intellectually acute as Holmes, and does not always understand what lies behind the events he observes and records.
Holmes: The consulting detective is brilliant and seeks and discovers the truth in all of the works featuring him. At times, he will lie or engage in other forms of deception to achieve his ultimate ends, but in the end he will reveal the truth.
Sir Henry Baskerville and Dr. Mortimer: Both honest but puzzled by events.
Barrymore and Mrs. Barrymore: Lie initially to help Selden but otherwise truthful. 
Mr. Stapleton: Not credible.

What information can be determined by observing the spacing of contour lines?

The information that can be determined by observing the spacing of contour lines on a topographic map is relative steepness.  It's relative steepness because without knowing the map's relief or the map's contour interval, all that a user can do is see that certain areas are steeper than others.  Contour lines that are closely spaced indicate a steeper section than contour lines that are widely spaced.  In order to know exactly how steep a particular slope is, the map user will need to know the contour interval.  The contour interval is the change in elevation from one contour line to the next contour line.  Once the elevation change is known, the user would need to know what the map's scale is. By knowing the contour interval and the map's scale, a person can then calculate the actual steepness of a slope. 
https://archive.cnx.org/resources/c1a53733e85627f1bff76acac1db3c42f5611efd/Picture%201.png

Does light affect the growth of an Elodea plant?

In general, plants need food in order to grow. The process by which plants make their own food is called photosynthesis. We can determine if light affects the growth of an Elodea plant by finding out the effect of light on its ability to produce its own food and maintain optimal growth.
Such experiments have been carried out using the Elodea plant before, and the results of one such experiment as presented in a research report by Dr Norman Herr indicates that light affects the rate at which the Elodea plant makes its own food and, by extension, its ability to maintain optimal growth.
The experiment concludes that excessive light may result in a ‘burn out,’ while too little light will not provide sufficient light for the plant to manufacture its own food and maintain optimal growth.
https://sites.google.com/site/sed695b3/projects/longitudinal-research/passive-transport-carol-cao

To what extent is Caesar admired?

After Caesar made his way back to Rome after his triumph in several wars, some welcomed him as a champion, while others feared him for his achievements. Part of the Senate and some members of the nobility were among those that feared him. They feared him because they believed that Caesar was attempting to become a tyrant. Those who feared him believed that their positions, influence, and wealth were at risk; others believed that the republic itself was at risk. The masses, on the other hand, loved him and venerated him as a result of his achievements. They saw him as a champion and did not mind having him as their supreme leader. Their love for him was evident after they hounded out all those who participated in Caesar’s assassination. Thus, Caesar was both admired and feared, depending on individual perceptions.

Why does Johnny react so violently to the Socs?

Johnny reacts so violently to the Socs because they are known for preying on Greasers. At the beginning of the book, we learn that Ponyboy was physically assaulted by members of the Socs, and Johnny himself has been a victim of the Socs' violent attacks. In chapter 2, Ponyboy tells the story of how some Socs members almost beat Johnny to death.
In Chapter 4, after Ponyboy's quarrel with Darry, Ponyboy goes to the park to cool off. He meets Johnny there, and before long, Randy and Bob (members of the Socs gang) corner the two. There are three other Socs with Randy and Bob. A Soc named David tries to drown Ponyboy in the fountain, and Ponyboy does his best to fight back.
However, Ponyboy is no match for David, and he soon suspects that he is going to die. As Ponyboy begins to lose consciousness, the pressure suddenly eases, and he finds himself on the ground beside the fountain. Ponyboy soon notices that Johnny seems dazed; the latter keeps telling Ponyboy that he's killed someone. 
It turns out that Johnny has killed Bob, one of the Socs. Johnny confesses that the Socs were going to drown Ponyboy and that he had to act before it was too late. So, Johnny reacts violently to the Socs because the gang regularly targets Greasers, especially if they are alone. He clearly remembers his attack at the hands of Socs months ago. Basically, Johnny's violent reaction stems from his fear that the Socs are aiming to kill him. 
 

Calculus of a Single Variable, Chapter 2, 2.3, Section 2.3, Problem 65

You need to evaluate the equation of the tangent line at (-5,5), using the formula:
f(x) - f(-5) = f'(-5)(x + 5)
Notice that f(-5) = 5.
You need to evaluate f'(x), using the quotient rule, such that:
f'(x) =((x)'(x+4) - (x)(x+4)')/((x+4)^2)
f'(x) = (x+4 - x)/((x + 4)^2)
f'(x) = 4/((x+4)^2)
You need to evaluate the derivative at x = -5:
f'(-5) = 4/((-5+4)^2) =>< f'(-5) = 4
Replacing the values into equation yields:
f(x) - 5= 4(x + 5)
f(x) = 4x + 25
Hence, evaluating the equation of the tangent line at the given curve, yields f(x) = 4x + 25.

The year is 1848. You are a member of one of the following classes: middle class, working class, or aristocrasy.You live in one of the following countries: England, France, or Austria.Discuss how the events from 1789 (the French Revolution) until 1848 (the end of the Age of Metternich) have affected you and your family.

This question gives you a lot of options. Given that you can choose to be in one of three classes from one of three countries, there are in fact nine different scenarios you could write about here. As such, it is best to let yourself be guided by the events and ideas defined in the question: the French Revolution and the Age of Metternich.
I assume you have been studying this in class. Do you have a preference for, or greater confidence about, the history of any one of these nations? If so, I would be inclined to choose that one. Personally, I think the most difficult of the countries to choose would be England because the question focuses upon political change in mainland Europe. While there were certainly significant changes to British society driven by the French Revolution, the subsequent unease about the rights of the monarchy, and the rigidity of the class system—not to mention the religious unease which followed—if you are struggling with this question, it might be better to focus on France, where the effects of the Revolution were much more clearly felt.
In terms of choosing an area where the effects would have been most extreme, it might be easier to focus on either the working or aristocratic class. The French Revolution saw a surge in the middle class in France, as social mobility became increasingly possible and the merchant class profited. However, the position of a person who was an aristocrat before the Revolution, provided that he or she survived the ordeal, would have been radically different after the execution of the King and Queen, and life over the next sixty years would have been a continuous period of adjustment. The same would be true for a person who had known absolute monarchy at the time of his or her birth into poverty but was now experiencing the radical overhaul of society.
Importantly, the time period defined in the question is bookended by two revolutions: the first French Revolution, and the February Revolution of 1848. A person who was a young adult in 1789 would have experienced the following:
1. Ten years of fighting and political turmoil, culminating in the overthrow of the French monarchy (to 1799), resulting in a secular society which had thrown out its Catholic roots and driven significant social change.
2. The expansion of the French Empire under Napoleon and the Napoleonic wars, with Napoleon as Emperor of France (1804–1815).
3. The restoration of the monarchy in 1814 but under different constraints to the original French monarchy. This monarchy was constitutional only—during this period, industrialization began in earnest in France and an economic recovery from the wars began.
4. The overthrow of Charles X by Louis Philippe in 1830, which saw a massive rise in the importance and political clout of the wealthy upper-middle class, plus colonial expansion for the French alongside many other European states.
5. Numerous revolts against the July monarchy, which ultimately resulted in the 1848 revolution. This created the Second Republic, which was driven by working-class opposition to the increasingly conservative monarchy.
That is an unprecedented amount of change in a very short period of time. It is part of what is often called France's long nineteenth century. As a member of a working-class family, how would all of these events and social changes have affected your life? Under the first monarchy, you would have effectively been a serf, probably an agricultural worker, undoubtedly a Roman Catholic, and at the bottom of the social pecking order with no political power. Over this period, you might find yourself fighting for Napoleon, becoming a factory worker and moving to a big city for work as industrialization picked up, or becoming politicized and joining a revolution (or several).
If you had been born into an aristocratic family, meanwhile, your circumstances would also have undergone radical change over this period. You might have seen members of your family executed, been forced to go into hiding, and watched society become increasingly secular. Society shifted from viewing the king as being ordained by God to a point where the power of the working classes could end monarchies (more than once). That is a busy life for anyone.
I hope this gives you a place to start!