Instructions: In this task, you're given the title of a five-sentence story, the first four sentences, and two options for the fifth sentence as a and b. Your job is to pick the sentence option that seamlessly connects with the rest of the story, indicating your choice as 'a' or 'b'. If both sentences are plausible, pick the one that makes more sense.
Input: Title: Bob Learns a Lesson. Sentence 1: Bob found pool maintenance to be a tiresome chore. Sentence 2: He had to add chlorine every week. Sentence 3: He wondered how a salt pool could stay clean without chlorine. Sentence 4: He learned that the engine of the salt pool separated elements. Choices: a. But after several months, she didn't improve. b. Of course, he understood because salt is sodium and chlorine.
Output:
b