instruction:
You are given a science question (easy-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.
question:
Which organ is made mostly of muscle?  
 (A) heart (B) lung (C) ear (D) brain
answer:
A


question:
Which characteristic of frogs is an adaptation to living in water?  
 (A) strong legs (B) webbed feet (C) dull colors (D) moist skin
answer:
B


question:
The frost line of the solar system divides the terrestrial plants from the gas-giant planets. What elements make up most of the core of the planets orbiting outside of the frost line?  
 (A) iron and nickel (B) carbon and oxygen (C) nitrogen and sulfur (D) hydrogen and helium
answer:

D