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]: Planting certain crop plants in a field causes a disruption of the natural nitrogen cycle in that field. Which outcome would this disruption most DIRECTLY cause in the field over time?  
 (A) topsoil erodes (B) soil nutrients decrease (C) desert conditions appear (D) atmospheric carbon dioxide increases
[EX A]: B

[EX Q]: When animals use energy, they produce  
 (A) nitrogen (B) iron sulfide (C) oxygen (D) carbon dioxide
[EX A]: D

[EX Q]: Gasoline is a nonrenewable energy resource used in most cars and trucks. Gasoline is made from which source?  
 (A) oil (B) hydrogen (C) natural gas (D) water vapor
[EX A]:
A