Q: 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.
When a car engine is started, gasoline is mixed with air and burned. Heat, sound, and chemical products are released. As the engine operates, which of these remains constant?
(A) the masses of compounds found in the engine (B) the amount of heat in the engine (C) the total mass of gasoline (D) the total amount of energy
A:
D