TASK DEFINITION: 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.
PROBLEM: Which of the following statements best explains how the decomposition of plant and animal matter helps to form rich soils?  
 (A) Decomposition adds silt to soil. (B) Decomposition adds water to soil. (C) Decomposition adds oxygen to soil. (D) Decomposition adds nutrients to soil.

SOLUTION: D

PROBLEM: 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

SOLUTION: B

PROBLEM: Edith's checkerspot butterfly is a non-migratory species found in distinct populations from Canada to Mexico. Today, populations in southern regions are declining while new populations push the northern limit further north. This range shift provides evidence for the theory of  
 (A) mass extinction. (B) global warming. (C) gene dominance. (D) natural selection.

SOLUTION:
B