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").
Q: Fact1: Mammals can also generate little bursts of heat by shivering., Fact2: Shivering costs the bird, however, by depleting energy stores., Question: How can mammals generate heat? (A) layers of fat (B) Ice cubes (C) wishing (D) Depleting energy stores (E) death (F) with their fur (G) raising their temperature (H) by keeping them warm
A:
D