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.

[EX Q]: Which force produces energy as heat?  
 (A) pull (B) gravity (C) friction (D) magnetic
[EX A]: C

[EX Q]: 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
[EX A]: A

[EX Q]: For half an hour, a student watches two different types of flowering bushes and counts the number of butterflies that come to feed on each bush. What would be the best way to show the data?  
 (A) a scatterplot (B) a pie chart (C) a bar graph (D) a line graph
[EX A]:
C