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: When microorganisms deplete the oxygen in soil through respiration, the soil becomes  
 (A) humus. (B) anaerobic. (C) compost. (D) aerobic.

Output: B


Input: Consider Input: Students are studying some leaves. They group the leaves by shape. Which science process are the students using when they put the leaves in different groups?  
 (A) classifying (B) predicting (C) measuring (D) experimenting


Output: A