TASK DEFINITION: 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.
PROBLEM: The ounce, the pound, and the ton are units of  
 (A) time. (B) weight. (C) length. (D) distance.

SOLUTION: B

PROBLEM: 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.

SOLUTION: C

PROBLEM: In what respect are protons and electrons always equal?  
 (A) mass number (B) kinetic energy (C) sign of charge (D) magnitude of charge

SOLUTION:
D