Given the task definition and input, reply with output. In this task, you're given five sentences, numbered 1 through 5, and two options a and b for possible titles for the story. Your job is to choose the title that better fits the story. Indicate your choice by 'a' or 'b'.

Sentence 1: Matilda wanted to throw a party. Sentence 2: However, she felt that her house was very messy. Sentence 3: She pulled out her broom, and started sweeping the floor. Sentence 4: To her surprise, the more she swept, the dirtier the floor became. Sentence 5: She quickly learned that the broom was covered in mud. Choices: a. Final Exam. b. Sweeping.
b