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: Barbara is a herpetologist, a scientist that studies reptiles. Once a week, she collects the venom from her captive rattlesnakes to make antivenin (medicine used to cure snakebites). Which of these would be the best estimate for the amount of venom collected from a snake? (A) 1.5 liters (B) 1.5 milliliters (C) 150 liters (D) 1500 milliliters?
Example Output: amount, venom, collected, snake?

Example Input: Question: The length of time from an animal's birth until its death is called its (A) life span. (B) instinct. (C) life cycle. (D) development.?
Example Output: length, time, animal's, birth, death

Example Input: Question: When does competition occur? (A) Only when the climate is hot (B) Only when the climate is cold (C) When food is plentiful (D) All of the time?
Example Output:
competition, occur?