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.

Example Input: Which is the best evidence that cell division is occurring constantly in our bodies?  
 (A) The body has to keep breathing around the clock. (B) Active people often require more food than inactive ones. (C) There are many different types of tissues in the human body. (D) Human beings shed millions of
Example Output: D

Example Input: Which body system's primary function is the continuation of the species?  
 (A) digestive (B) nervous (C) excretory (D) reproductive
Example Output: D

Example Input: Heat was applied to a substance. First the substance expanded, and then it melted. Which state of matter did the substance start off as?  
 (A) Gas (B) Solid (C) Liquid (D) Plasma
Example Output:
B