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.

Input: Consider 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?

Output: explains, hears, shout?


Input: Consider Input: Question: The skin of a chameleon can change colors to help it blend with its surroundings. How would this adaptation most likely help a chameleon survive? (A) by scaring predators away from the chameleon (B) by attracting prey animals to the chameleon (C) by sending a signal to other chameleons (D) by helping the chameleon hide from its predators?

Output: skin, chameleon, change, colors, blend, adaptation, chameleon, survive?


Input: Consider Input: Question: Josh designed an experiment to see how friction affects the distance a ball travels. What should Josh change in his experiment? (A) Force applied to the ball (B) Size of the ball (C) Type of surface the ball rolls on (D) Type of ball?


Output: friction, affects, distance, travels., change, experiment?