Instructions: In this task, you are given two facts, and a multiple-choice question. Based on the given facts, answer the question with index of the correct option (e.g, "A").
Input: Fact1: Muscle contraction occurs when muscle fibers get shorter., Fact2: Muscle activation is a prerequisite for muscle contraction., Question: what happens to fibers after muscle activation? (A) It expands (B) Localized damage (C) It decreases (D) get shorter (E) they bend (F) rapid expansion (G) sweating (H) movement
Output:
D