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: Darwin's theory that animal species can change over time was inspired by his research on which set of islands? (A) the Philippine Islands (B) the Virgin Islands (C) the Hawaiian Islands (D) the Galapagos Islands?

Ex Output:
Darwin's, theory, species, time, inspired, research, islands?

Ex Input:
Question: Students are studying liquids that make bubbles. Which question could be answered by using a ruler? (A) Which liquid makes bubbles that are the largest? (B) Which liquid makes bubbles that are the prettiest? (C) Which liquid makes bubbles that are the best smelling? (D) Which liquid makes bubbles that are the most interesting?

Ex Output:
liquids, bubbles., answered, using, ruler?

Ex Input:
Question: Which of these activities is used to conserve water? (A) planting crops that survive in droughts (B) running water while brushing teeth (C) washing vehicles frequently (D) watering grass after a rainfall?

Ex Output:
activities, used, conserve, water?