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: a valley is formed by a river flowing, Fact2: Rivers erode the rocks they flow over., Question: What is created when rocks erode? (A) calcite (B) A pond (C) A valley (D) detritus (E) fossil fuels (F) A river (G) A volcano (H) a prism
A:
C