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").
Example: Fact1: a plant requires sunlight to grow, Fact2: Plants to avoid are fast-growing vines that can take over an area in no time., Question: What requires sunlight in order to take over an area? (A) spores (B) fish (C) plants (D) humans (E) pods (F) Oak (G) opossums (H) coral
Example solution: C
Example explanation: This is a good example. Based on the facts plants require sunlight in order to take over an area.

Problem: Fact1: soil is formed by rocks eroding, Fact2: Rivers erode the rocks they flow over., Question: What is formed by rivers? (A) fish (B) soil (C) rain (D) bamboo (E) spring (F) clouds (G) kayaks (H) NaCl

Solution: B