TASK DEFINITION: You are given a science question (hard-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: What part of the digestive system first causes chemical changes to food?  
 (A) teeth in the mouth (B) saliva in the mouth (C) enzymes in the stomach (D) enzymes in the small intestine

SOLUTION: B

PROBLEM: How are green plants an important part of the carbon dioxide-oxygen cycle?  
 (A) They add oxygen to the soil. (B) They release oxygen into the air. (C) They fix carbon dioxide in the soil. (D) They store carbon dioxide in roots.

SOLUTION: B

PROBLEM: The structures found in a living cell can be compared to the parts of a factory that produces cars. Which part of the factory is most similar to the nucleus of a living cell?  
 (A) a conveyor belt that transports materials (B) a storage bin that holds the pieces needed to assemble a car (C) the computer room that controls the assembly process (D) the generator that provides energy for the factory

SOLUTION:
C