instruction:
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.
question:
Question: When ice cream is left out of a freezer, the ice cream changes from a (A) solid to a gas (B) gas to a liquid (C) solid to a liquid (D) liquid to a gas?
answer:
ice, cream, left, changes


question:
Question: Which of the following environments would be the BEST suited for an amphibian? (A) desert (B) arctic (C) tropical (D) savanna?
answer:
environments, BEST, amphibian?


question:
Question: Why are invasive new species so harmful to the stability of an ecosystem? (A) They do not have any natural predators. (B) They all produce too much carbon monoxide. (C) They all give off toxins in the environment. (D) They do not have any food to eat.?
answer:

invasive, species, harmful, ecosystem?