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.

Ex Input:
Question: What is the main source of heat energy on Earth? (A) fire (B) volcanoes (C) the sun (D) electricity?

Ex Output:
main, source, heat, energy, Earth?


Ex Input:
Question: A student riding a bicycle observes that it moves faster on a smooth road than on a rough road. This happens because the smooth road has (A) less gravity(B) more gravity(C) less friction(D) more friction?

Ex Output:
faster, smooth, rough, smooth


Ex Input:
Question: One thing that happens when overpopulation occurs in an area is ___. (A) climate change (B) the extinction of at least one species (C) an increase in the use of resources (D) an increase in the availability of resources?

Ex Output:

happens, overpopulation, area