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.

Ex Input:
Which substance is a compound?  
 (A) butter (B) nitrogen (C) hydrogen (D) ammonia

Ex Output:
D


Ex Input:
Which characteristic property indicates that igneous rock cools slowly?  
 (A) mineral composition (B) hardness (C) density (D) crystal size

Ex Output:
D


Ex Input:
The respiratory system cannot deliver oxygen to the cells of the body, nor can it remove carbon dioxide, without which two body systems?  
 (A) muscular and nervous (B) digestive and muscular (C) cardiovascular and nervous (D) cardiovascular and muscular

Ex Output:

D