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.

Compost piles have many parts, such as leaves, moisture, vegetables, and microorganisms. Which part of a compost pile breaks down materials and adds nutrients to the soil?  
 (A) leaves (B) moisture (C) vegetables (D) microorganisms
D

Which substance do all living things require to survive?  
 (A) blood (B) water (C) carbon dioxide (D) organic soil
B

What is the name of the process in which igneous, sedimentary, and metamorphic rocks are converted into one another in stages?  
 (A) rock cycle (B) erosion cycle (C) magma cycle (D) tectonic cycle

A