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 question will best help a student determine the composition of an unknown substance?  
 (A) Is the sample warm or cold? (B) How valuable is the sample? (C) Has the sample ever been melted? (D) Does the sample conduct electricity?
Example Output: D

Example Input: Several species of fish live in a classroom aquarium. Students notice that the gills of one type of fish move faster than the gills of the other fish. Which of these statements is the best hypothesis on this observation?  
 (A) Fish have a two-chambered heart. (B) Some fish are much larger than others. (C) Fish need different amounts of oxygen. (D) There are six types of fish in the aquarium.
Example Output: C

Example Input: The salt in ocean water comes from all of the following except  
 (A) eroding land. (B) melting glacial ice. (C) volcanic emissions. (D) reactions on the sea floor.
Example Output:
B