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: Gene flow occurs when individuals move into or out of a population., Fact2: Gene flow tends to decrease differentiation., Question: What tends to occur when individuals move in and out of a population? (A) By wind and rain (B) permineralization (C) The bending of a ray of light (D) Something with a head, thorax, and abdomen (E) a decrease in differentiation (F) competing for resources (G) barks and snarls (H) metabolic reaction
E