Definition: 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'.
Input: Sentence 1: Ethan's friends challenged him to drink 2 liters of cola in an hour. Sentence 2: They pooled their funds and offered him twenty dollars. Sentence 3: Ethan accepted the challenge. Sentence 4: He struggled, but he managed to drink all the cola on time! Sentence 5: However, that night Ethan had a terrible stomach ache! Choices: a. Challenge. b. Slow in understanding.

a