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 type of gas do plants use to make their own food? (A) oxygen (B) nitrogen (C) carbon dioxide (D) hydrogen?

Output: gas, plants, food?


Input: Consider Input: Question: Which activity is an example of a chemical change? (1) dissolving table salt in water (2) hammering aluminum into thin sheets (3) melting gold to make jewelry (4) burning wood to produce ashes?

Output: example, chemical, change?


Input: Consider Input: Question: The aloe plant can absorb a lot of water during a rain shower. The extra water is stored in its leaves. The ability to store water in its leaves is most likely an adaptation to which type of environment? (A) one near the ocean (B) one with dry conditions (C) one with a variety of organisms (D) one that receives a lot of sunlight?


Output: aloe, water, rain, shower., extra, water, leaves., store, water, adaptation, environment?