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: All of the different organisms interacting in a pond make up  
 (A) a community (B) a population (C) the water cycle (D) the habitat

SOLUTION: A

PROBLEM: Fertilizers can enable farmers to grow the same crop in a field for several years in a row. Farmers who use less fertilizer often rotate their crops by planting the crop one year and legumes, such as beans and clover, the following year. Fertilizer use and crop rotation with legumes both increase the availability of which of the following nutrients in soil?  
 (A) calcium (B) nitrogen (C) oxygen (D) protein

SOLUTION: B

PROBLEM: When an atom loses or gains an electron, it changes  
 (A) atomic mass. (B) atomic number. (C) charge. (D) element.

SOLUTION:
C