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: Behaviors that improve fitness increase through natural selection., Fact2: Natural selection is the mechanism of evolution., Question: What process increases behaviors that improve fitness? (A) competition (B) sweating (C) humanization (D) evolution (E) movement (F) photosynthesis (G) sleep (H) cats
A:
D