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: A squid is an animal that lives in the ocean. It pumps a stream of water out of its body, causing it to move rapidly. Which of the following mechanical systems moves in a way most similar to the squid? (A) a bus (B) a helicopter (C) a rocket (D) a train?
Example Output: squid, pumps, water, mechanical, systems, similar

Example Input: Question: A student in an empty classroom shouts, "Hello!" Which best explains what the student hears after the shout? (A) an increased loudness of sound (B) a reflection of sound (C) an increased frequency of sound (D) a refraction of sound?
Example Output: explains, hears, shout?

Example Input: Question: The picture below shows a stream flowing over rocks. How do the water and rocks interact? (A) The water evaporates into the air because of the position of the rocks. (B) The water soaks into the ground beneath the rocks. (C) The water changes the shape of the rocks. (D) The water floods over its banks because of the mass of the rocks.?
Example Output:
stream, water, rocks, interact?