Definition: 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: a valley is formed by a river flowing, Fact2: Fast flowing rivers tend to pick up more pebbles, sand and other materials that cause erosion., Question: How does water cause a valley to form? (A) Physical weathering (B) By the process of erosion (C) By falling from clouds (D) It used to be covered by water (E) fissures of hot, acidic water (F) Heavy snow accumulation (G) By forming puddles (H) Water expanding
Output:
B