instruction:
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.
question:
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
answer:
B


question:
Auroras occur where Earth's magnetosphere traps high-energy particles in which region of the atmosphere?  
 (A) troposphere (B) stratosphere (C) thermosphere (D) exosphere
answer:
C


question:
A vaccine can prevent a serious disease. When a new vaccine is developed, it is given to a small group of people. Why would it be important to test each vaccine on a small group of people before everyone can have one?  
 (A) to figure out how much the vaccine is worth (B) to determine how the vaccine will affect people (C) to understand how that disease affects the human body (D) to find out how many people are affected by the disease
answer:

B