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: Paul is waiting for the traffic light to change so he can cross the street. What form of energy is present in Paul as he waits to cross the street? (A) Mechanical (B) Muscular (C) Potential (D) Kinetic?
Example Output: waiting, form, energy, present

Example Input: Question: Martin is writing a report for science class about how the tilt of Earth affects living organisms. Which subject should Martin include in his report that is directly affected by Earth's tilt? (A) night and day (B) cloud formation (C) phases of the Moon (D) changing of the seasons?
Example Output: Earth, living, organisms., affected, tilt?

Example Input: Question: Which of the following is likely to cause the most erosion? (A) a river (B) a small creek (C) a pond (D) a puddle?
Example Output:
cause, erosion?