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.
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Question: When do mutations help make an ecosystem more sustainable?  
 (A) when mutations decrease the number of individuals in an ecosystem (B) when mutations increase the number of individuals in an ecosystem (C) when mutations decrease the number of species in an ecosystem (D) when mutations increase the number of species in an ecosystem

Answer: D


Question: Which of the following best represents one particular stage in a life cycle?  
 (A) a fish swimming (B) a seed sprouting (C) a leaf growing (D) a dog eating

Answer: B


Question: What is the body's first layer of defense against germs?  
 (A) liver (B) stomach (C) brain (D) skin

Answer:
D