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.

Example Input: Question: The Sun appears to move across the sky each day. What causes this?
Example Output: Sun, appears, move, sky, causes

Example Input: Question: A student found a rock while hiking in the mountains. By looking at the rock, she could tell the (A) exact weight of the rock. (B) length of time the rock had been on the hiking path. (C) color and shape of the rock. (D) exact length of the rock.?
Example Output: looking, tell

Example Input: Question: What is the purpose of preforming an experiment? (A) to test a hypothesis (B) to make a measurement (C) to grow a plant (D) to remain safe?
Example Output:
purpose, experiment?