You are given a science question (easy-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.

[EX Q]: The shape of a riverbed changes over time as a result of which gradual process?  
 (A) erosion (B) dissolving (C) plate shifting (D) compaction
[EX A]: A

[EX Q]: Which of the following would be more likely to pollute a river?  
 (A) allowing beavers to build a dam (B) spreading fertilizer near the river (C) mowing the grass on the river bank (D) fishing from a bridge over the river
[EX A]: B

[EX Q]: When a formula unit of magnesium sulfide, MgS, is separated into magnesium and sulfur particles, the particles are called  
 (A) molecules. (B) ions. (C) isotopes. (D) compounds.
[EX A]:
B