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.

Which two protein molecules are essential for muscle contraction and relaxation?  
 (A) actin and myosin (B) collagen and tubulin (C) hemoglobin and myoglobin (D) amylase and polymerase
A

Where do good bacteria grow to help keep people healthy?  
 (A) in the brain (B) in the blood (C) on the skin (D) on the bones
C

Which of the following is a reason life in the outermost planets is less likely than life on planets nearer the Sun?  
 (A) They have too little mass. (B) They have too much mass. (C) Their surfaces are extremely hot. (D) Their surfaces are extremely cold.

D