instruction:
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.
question:
Which involves the greatest variety of cell types?  
 (A) a tissue (B) an organ (C) an organ system (D) an organism
answer:
D


question:
Where do good bacteria grow to help keep people healthy?  
 (A) in the brain (B) in the blood (C) on the skin (D) on the bones
answer:
C


question:
What is an example of a force producing heat?  
 (A) an apple falling from a tree branch (B) doorbell ringing when a switch is pushed (C) a wagon rolling across a yard when pulled (D) two sticks getting warm when rubbed together
answer:

D