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]: Proxima Centauri is the closest star to the Sun. Which units do astronomers most often use for the distance between Earth and Proxima Centauri?  
 (A) miles (B) meters (C) light years (D) astronomical units
[EX A]: C

[EX Q]: Which of the following climates has cold winters and hot summers?  
 (A) polar (B) subtropical (C) temperate (D) tropical
[EX A]: C

[EX Q]: Which of the following keeps the planets in our solar system in orbit around the Sun?  
 (A) atmospheric pressure (B) gravitational force (C) electromagnetic energy (D) thermal energy
[EX A]:
B