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: Two atoms of oxygen combine to form pure oxygen gas. What is the electrical charge of the O_{2} molecules?  
 (A) positive (B) negative (C) neutral (D) variable

A: C
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Q: In many caves, rock formations include stalactites and stalagmites. Where does the rock in these formations come from?  
 (A) magma (B) rainwater (C) dissolved minerals (D) decaying organic matter

A:
C
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