You are given a science question (hard-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]: Two forms of boron are naturally occurring, boron-10 and boron-11. What difference exists between a boron atom with a 10 atomic mass units (amu) and a boron atom with 11 atomic mass units (amu)?  
 (A) amount of neutrons (B) number of electrons (C) size of protons and neutrons (D) size of electrons and electron cloud
[EX A]: A

[EX Q]: As a car burns fuel, a transformation from chemical to mechanical energy takes place. What is the most likely result of the fuel burning?  
 (A) The matter that makes up the car increases. (B) The amount of useful energy decreases. (C) The efficiency of the car increases. (D) The amount of heat energy decreases.
[EX A]: B

[EX Q]: When Jake breathes out on a cold day, a cloud is formed. Which change occurs that lets Jake see his breath as a cloud?  
 (A) Gas changes to liquid. (B) Solid changes to gas. (C) Liquid changes to gas. (D) Liquid changes to solid.
[EX A]:
A