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.

[EX Q]: Human DNA includes a class of genes called "tumor suppressors." What do these genes do in bone cells?   
 (A) They restrict cell cycle progression in bone cells. (B) They promote the rapid development of bone cells. (C) They limit the number of bones in the human body. (D) They enable phosphorus and calcium to enter bone cells.
[EX A]: A

[EX Q]: What is a main feature of plains?  
 (A) a flat, level surface (B) a high, steep peak (C) a low, shallow basin (D) a gentle, gradual slope
[EX A]: A

[EX Q]: Which process causes the fastest changes to the surface of Earth?  
 (A) a large glacier forming on a mountain (B) strong winds wearing away a rock cliff (C) a major earthquake happening in a mountain range (D) many waves on a lake moving pebbles onto a beach
[EX A]:
C