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.

[EX Q]: When microorganisms deplete the oxygen in soil through respiration, the soil becomes  
 (A) humus. (B) anaerobic. (C) compost. (D) aerobic.
[EX A]: B

[EX Q]: Which of the following units best represents the density of an object?  
 (A) kg (B) hr (C) m/s^2 (D) g/cm^3
[EX A]: D

[EX Q]: Which features form when magma emerges between two diverging oceanic plates?  
 (A) ocean trenches (B) fault boundaries (C) mid-ocean ridges (D) composite volcanoes
[EX A]:
C