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: The body is made up of many systems that interact with one another. For example, the skeletal system and the muscular system work together to help the body move. Which system works with the respiratory system to distribute oxygen to the body?  
 (A) muscular system (B) digestive system (C) lymphatic system (D) circulatory system

A: D
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Q: Which of the following is an example of a behavioral adaptation?  
 (A) hooves of a horse (B) migration of birds (C) a spider web (D) a bee hive

A: B
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Q: Plants growing in a pond release a gas that fish need to survive. Which gas do the plants release that fish use?  
 (A) helium (B) hydrogen (C) nitrogen (D) oxygen

A:
D
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