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

Which particle type can be identified as being from a specific element?  
 (A) electron (B) atom (C) neutron (D) proton
B

When the fuel in a gasoline engine is burned, the energy conversion is from  
 (A) chemical to thermal. (B) potential to nuclear. (C) thermal to potential. (D) electrical to kinetic.

A