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.

[EX Q]: You wish to closely observe a small insect. Which tool is the best to use?  
 (A) a balance (B) a test tube (C) a telescope (D) a magnifying glass
[EX A]: D

[EX Q]: Which two body systems maintain the level of glucose in the blood?  
 (A) nervous and respiratory systems (B) digestive and endocrine systems (C) muscular and immune systems (D) skeletal and cardiac systems
[EX A]: B

[EX Q]: Why has only one-half of the Moon ever been observed from Earth?  
 (A) The Moon does not rotate on its axis. (B) The Moon is not visible during the day. (C) The Moon has phases that coincide with its rate of rotation. (D) The Moon rotates at the same rate that it revolves around Earth.
[EX A]:
D