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]: An object such as a hammer weighs less on the Moon than on Earth. This is because the Moon has a  
 (A) thinner atmosphere than Earth. (B) smaller diameter than Earth. (C) stronger magnetic field than Earth. (D) lower mass than Earth.
[A]: D


[Q]: Which two body systems are involved in inhaling oxygen and exhaling carbon dioxide?  
 (A) endocrine and skeletal (B) excretory and nervous (C) circulatory and respiratory (D) digestive and reproductive
[A]: C


[Q]: What causes day and night on Earth?  
 (A) Earth's rotation on its axis (B) the Sun's revolution around Earth (C) Earth's revolution around the Sun (D) the Sun's rotation on its axis
[A]:
A