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.

Example Input: Rocks are classified as igneous, metamorphic, or sedimentary according to  
 (A) their color (B) their shape (C) how they formed (D) the minerals they contain
Example Output: C

Example Input: 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.
Example Output: A

Example Input: In science, old ideas can be changed when new discoveries alter existing information. Which situation is most likely an example of a rethinking of an existing idea?  
 (A) using coal to generate electricity (B) discovering a new species of snake (C) developing an antibiotic to treat infections (D) changing the definition of the term planet
Example Output:
D