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: 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

SOLUTION: B

PROBLEM: Which characteristic of frogs is an adaptation to living in water?  
 (A) strong legs (B) webbed feet (C) dull colors (D) moist skin

SOLUTION: B

PROBLEM: How does a mushroom obtain energy?  
 (A) by attaching to green plants and taking the plants' food (B) by combining carbon dioxide and water to make its own food (C) by absorbing matter from dead organisms to use as food (D) by attacking bacteria and using the bacteria as food

SOLUTION:
C