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: What is the main function of red blood cells?  
 (A) To fight disease in the body (B) To carry oxygen to all parts of the body (C) To remove carbon monoxide from all parts of the body (D) To produce materials which cause the blood to clot.

SOLUTION: B

PROBLEM: Which force produces energy as heat?  
 (A) pull (B) gravity (C) friction (D) magnetic

SOLUTION: C

PROBLEM: Most birds sit on their eggs until they hatch. Which of these is the most important reason why birds sit on their eggs?  
 (A) to keep the eggs inside the nest (B) to keep the eggs warm (C) to protect the eggs from the wind (D) to protect the eggs from the rain

SOLUTION:
B