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 type of organism is the best at fixing nitrogen?  
 (A) bacteria (B) fungi (C) protists (D) animals

A: A
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Q: Which of the following compounds is most likely to be part of living organisms?  
 (A) C?H??O? (B) BF? (C) MoCl? (D) CsI

A: A
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Q: Over a long time, oil can form when layers of sediment push down on decomposing  
 (A) trees. (B) animals. (C) fungi. (D) algae.

A:
D
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