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: Some birds fly south in the fall and return in the spring. This is an example of  
 (A) migration (B) camouflage (C) hibernation (D) growth

A: A
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Q: Which statement correctly describes a relationship between two human body systems?  
 (A) The digestive system absorbs nutrients which are transported by the nervous system. (B) The digestive system absorbs nutrients which are transported by the circulatory system. (C) The circulatory system exchanges gases which are transported by the nervous system. (D) The circulatory system exchanges gases which are transported by the digestive system.

A: B
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Q: Census data show that the human population doubled from 2.65 billion in 1953 to 5.3 billion in 1990. If it continues to double at this rate, the human population could reach 10 billion by the year 2020. Which best explains how this rate of human growth can harm the environment on which humans depend?  
 (A) by gradually making up more available energy (B) by using resources faster than they can be replaced (C) by changing how matter cycles through an ecosystem (D) by increasing the variety of ecosystems that are supported

A:
B
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