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.

What force pulls things towards the center of Earth?  
 (A) magnetism (B) gravity (C) friction (D) energy
B

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

Which type of ecosystem has the greatest variation in temperature from its top to its bottom?  
 (A) lake (B) pond (C) ocean (D) estuary

C