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 Q]: Question: What is the major function of the human excretory system? (1) eliminate waste materials (2) respond to stimuli (3) break down food mechanically (4) absorb nutrients from food?
[EX A]: function, excretory, system?

[EX Q]: Question: What happens at the boiling point of a substance? (A) liquid becomes a gas (B) solid becomes a gas (C) liquid becomes a solid (D) solid becomes a liquid?
[EX A]: boiling, point

[EX Q]: Question: Which of these is most likely caused by the action of water, wind, and animals? (A) mountains (B) erosion (C) earthquakes (D) volcanic eruptions?
[EX A]:
caused, action, animals?