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]: All of the following can become fossils except  
 (A) bones. (B) shells. (C) teeth. (D) rocks.
[EX A]: D

[EX Q]: The gravitational force between two objects depends on the distance between the objects and each object's  
 (A) mass (B) volume (C) pressure (D) temperature
[EX A]:
A