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.

Input: Consider Input: 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.

Output: C


Input: Consider Input: Which characteristic best identifies a pure substance?  
 (A) shape (B) texture (C) melting point (D) state of matter

Output: C


Input: Consider Input: Students are attempting to make a doorbell ring using some wire and a battery. What is needed to make the bell ring?  
 (A) friction (B) gravity (C) a convection current (D) an electrical circuit


Output: D