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: If a child scrapes his foot on a piece of rusty metal, it is important to get medical treatment because the  
 (A) metal is corrosive to the skin. (B) foot carries large amounts of bacteria. (C) foot is the most sensitive part of the body. (D) metal may be contaminated and cause infection.
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: What does photosynthesis produce that helps plants grow?  
 (A) water (B) oxygen (C) protein (D) sugar
Example Output:
D

