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.

[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.
[A]: C


[Q]: Which location on Earth receives the most direct sunlight?  
 (A) the deserts (B) the South Pole (C) the equator (D) the Western Hemisphere
[A]: C


[Q]: Xylem cells have very thick walls. They make up most of a tree trunk and its branches. What system in vertebrates provides the same function as the xylem cells in a tree?  
 (A) the skeletal system (B) the endocrine system (C) the respiratory system (D) the integumentary system
[A]:
A