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 energy that is stored in food originally comes from  
 (A) air (B) soil (C) sunlight (D) water

SOLUTION: C

PROBLEM: Which part of an atom is involved in chemical reactions?  
 (A) nucleus and its particles (B) negatively-charged particles (C) neutrally-charged particles (D) positively-charged particles

SOLUTION: B

PROBLEM: A change in the environment that causes a response is known as a  
 (A) stimulus (B) habit (C) reflex (D) source

SOLUTION:
A