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]: Scientists have studied the specific stages in the life cycle of stars. In which stage do stars spend most of their lifetime?  
 (A) red giant (B) black dwarf (C) white dwarf (D) main sequence
[EX A]: D

[EX Q]: In order to maintain good health, humans should  
 (A) eat foods high in fat (B) exercise regularly (C) drink alcohol often (D) smoke tobacco occasionally
[EX A]: B

[EX Q]: Planets in the solar system are in constant motion. What factor has the greatest effect on the orbits of the planets?  
 (A) the size of the planets (B) gravitational pull of the Sun (C) the composition of the planets (D) electromagnetic radiation from the Sun
[EX A]:
B