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: Which unit is used to indicate the amount of energy in food?  
 (A) ounce (B) degree (C) calorie (D) gram

SOLUTION: C

PROBLEM: What is the most likely result of large-scale agricultural fertilization on aquatic systems?  
 (A) increased fish populations (B) more biodiversity over time (C) less oxygen available for fish (D) reduced water levels

SOLUTION:
C