Given the task definition and input, reply with output. 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").

Fact1: Behaviors that improve fitness increase through natural selection., Fact2: Many organism behaviors have evolved through natural selection., Question: What is one of the facets of an organism's evolution? (A) resistance (B) fitness (C) properties (D) thirst (E) movement (F) swimming (G) offspring (H) recycling
B