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.

Example Input: Question: Carbon dioxide is necessary for photosynthesis to happen. How do plants absorb carbon dioxide from the atmosphere? (A) Through the petals (B) Through the stigma (C) Through the stomata (D) Through the stem?
Example Output: plants, absorb, carbon, dioxide, atmosphere?

Example Input: 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?
Example Output: boiling, point

Example Input: Question: Protein is used by the human body to (A) build strong bones. (B) absorb vitamins. (C) repair cells. (D) provide fiber.?
Example Output:
Protein, used, human, body