Which sentence from the story explains Miss Strangeworth's motivation for sending the letters?

According to the story, Miss Strangeworth “never knows when she might feel like writing letters” so she keeps her notepaper close by, locked in a “narrow desk” in a corner of her sitting room. This sentence seems to imply that Mrs. Strangeworth writes the letters whenever she feels the desire to. Another sentence states that “for as long as evil exists unchecked in the world, it was Miss Strangeworth’s duty to keep her town alert to it.” This implies that Miss Strangeworth’s anonymous letters to the townspeople are meant to warn them of possible problems so that they can solve these problems before it is too late—quite like a stitch in time saves nine.
Miss Strangeworth has been writing letters to the people of the town for over a year. Her letters are based on rumors or suspicions, rather than facts. Since she, and her family before her, have lived in the town for a long time, she feels like she owns the town. She thinks that her letters can help to straighten up things that might go wrong in households in the area. Even though her letters are malicious and hurtful, she claims that “a clean heart is a scoured heart.” Unfortunately, one of her letters falls off the mail slot and is picked by the Harris boy, who thinks to hand deliver it to its recipient, Don Crane. Through this single act of kindness, the Harris boy unknowingly helps to demystify the anonymous letters and also unveils Miss Strangeworth’s evil nature.