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.

Rocks are classified as igneous, metamorphic, or sedimentary according to  
 (A) their color (B) their shape (C) how they formed (D) the minerals they contain
C

What is the primary function of white blood cells in humans?  
 (A) carry oxygen (B) fight disease (C) grow tissues (D) remove waste
B

Coal, oil, and natural gas have high potential energies, which makes them valuable energy resources. In order to benefit from the energy available in these fossil fuels, the potential energy is converted most directly into which type of energy?  
 (A) chemical (B) electrical (C) solar (D) thermal

D