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: Which statement best explains Earth's day and night cycle?

Output: explains, Earth's, day, night, cycle?


Input: Consider Input: Question: A ball is dropped from the top of a tall building. As the ball falls, the upward force of air resistance becomes equal to the downward pull of gravity. When these two forces become equal in magnitude, the ball will?

Output: air, resistance, equal, gravity., forces, equal


Input: Consider Input: Question: The wheels and gears of a machine are greased in order to decrease (1) potential energy (2) efficiency (3) output (4) friction?


Output: wheels, gears, greased, decrease