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: Which of the following manmade structures are built to stop flooding? (A) Forests (B) Dams (C) Skyscrapers (D) Bridges?
[EX A]: manmade, structures, stop, flooding?

[EX Q]: Question: In addition to oxygen, which gases make up the largest percentage of Earth's atmosphere? (1) hydrogen, helium, and water vapor (2) hydrogen, methane, and ozone (3) carbon dioxide, water vapor, and nitrogen (4) carbon dioxide, methane, and helium?
[EX A]: gases, percentage, Earth's, atmosphere?

[EX Q]: Question: Which of the following is an example of an igneous rock? (A) granite (B) marble (C) sandstone (D) coal?
[EX A]:
example, igneous, rock?