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.

Q: Which of the following statements best explains how the decomposition of plant and animal matter helps to form rich soils?  
 (A) Decomposition adds silt to soil. (B) Decomposition adds water to soil. (C) Decomposition adds oxygen to soil. (D) Decomposition adds nutrients to soil.

A: D
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Q: Which of these is a structure that is present in all organisms?  
 (A) a tissue (B) a cell (C) a stomach (D) a brain

A: B
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Q: Imprints of the shells of ocean clams are often found in the rocks of the Appalachian Mountains. What do these imprint fossils most likely indicate?  
 (A) These mountains were once under the ocean. (B) These mountains once had a more rainy climate. (C) Mountain river clams evolved from ocean clams. (D) Ocean clams evolved from mountain river clams.

A:
A
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