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]: All of the following are functions of a cell membrane except  
 (A) surrounding and protecting the contents of the cell. (B) releasing waste products from the cell. (C) producing energy for cell processes. (D) allowing gases to enter the cell.
[EX A]: C

[EX Q]: When manned space exploration began, NASA used the gravity assist concept so that spacecraft could conserve fuel. Which statement explains why the gravity assist concept was useful for spacecraft to successfully travel and return to Earth?  
 (A) It gave spacecraft a timed trajectory. (B) It ensured spacecraft would return home safely. (C) It was helpful in planning spacecraft trajectories. (D) It ensured spacecraft would avoid being pulled into the sun.
[EX A]: C

[EX Q]: What keeps the planets in orbit around the Sun?  
 (A) tides (B) gravity (C) electricity (D) magnetism
[EX A]:
B