You are given an elementary science question along with a few answer options. List all the words from the question that are essential and sufficient for choosing the correct answer option. Keep in mind that essential words are necessary to understand the meaning of the given question. However, non-essential words provide little information needed to select the correct answer. You can think about it like this: If the question was in another language, knowing the meaning of which words were necessary and enough for you to understand the question and answer it?. If your answer consists of multiple spans of words from the question, separate those using a comma (", "). Do not generate any word that is not present in the question. Make sure that your response is contained in the given question and the words are in the same order as the question.
 Grammatical correctness is not important.

[Q]: Question: What is the main source of heat energy on Earth? (A) fire (B) volcanoes (C) the sun (D) electricity?
[A]: main, source, heat, energy, Earth?


[Q]: Question: Naturally occurring variations within a species are mainly the result of mutations and (1) sexual reproduction (2) dynamic equilibrium (3) metabolism (4) camouflage?
[A]: variations, species, result, mutations


[Q]: Question: Some of the new cars being made can run on a renewable fuel that is made mostly from corn. Scientists are developing even more renewable fuels to replace gasoline. Why are renewable fuels most likely becoming more important? (A) Renewable fuels release less energy. (B) Car fuel tanks are being made smaller. (C) The demand for gasoline is decreasing. (D) Nonrenewable fuel supplies are limited.?
[A]:
renewable, fuels, important?