Instructions: 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").
Input: Fact1: contact between rocks over long periods of time causes rocks to smooth, Fact2: Gravity can exert forces that cause friction in rocks and ice., Question: What can friction due to gravity cause? (A) smoothing (B) energy (C) heat energy (D) waves (E) erosion (F) sweating (G) an object (H) Energy.
Output:
A