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: How many elements are in the compound Mg(OH)2?   
 (A) 2 (B) 3 (C) 4 (D) 5
Example Output: B

Example Input: Which organ in a frog has a function similar to the function of lungs in a bird?  
 (A) kidney (B) skin (C) liver (D) heart
Example Output: B

Example Input: Which of the following statements best explains why the lower mantle of Earth is much more rigid and dense than the upper mantle?  
 (A) The lower mantle is older than the upper mantle. (B) The lower mantle is cooler than the upper mantle. (C) The lower mantle is under more pressure than the upper mantle. (D) The lower mantle is farther from the core than the upper mantle.
Example Output:
C