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.

[Q]: The cheetah population around the world has decreased because of poaching and loss of habitat. The surviving population is found to have similar alleles. Which factor is endangering the species' survival the most?  
 (A) harmful genetic mutations (B) shrinking of the gene pool (C) decrease in competition (D) increase in adaptations
[A]: B


[Q]: Which two body systems maintain the level of glucose in the blood?  
 (A) nervous and respiratory systems (B) digestive and endocrine systems (C) muscular and immune systems (D) skeletal and cardiac systems
[A]: B


[Q]: Which atmospheric gas has the greatest effect on the weather conditions associated with an air mass?  
 (A) oxygen (B) nitrogen (C) methane (D) water vapor
[A]:
D