Q: 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: contact between rocks over long periods of time causes rocks to smooth, Fact2: First, limestone is a strong rock., Question: What causes limestone to smooth? (A) contact (B) coffin (C) hair (D) erosion (E) bacon (F) rainfall (G) friction (H) loose soil
A:
A