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]: What are produced in both the male and female reproductive systems in humans?
(A) sperm (B) nutrients (C) fertilized eggs (D) sex cells
[EX A]: D

[EX Q]: Roger poured water over a pile of sand. Some of the sand washed away. This process is similar to which of the following?
(A) The eruption of a volcano (B) The erosion of the walls of a canyon (C) The uplifting of mountain ranges (D) The forming of dunes or mounds in a desert
[EX A]:
B