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: Students are studying liquids that make bubbles. Which question could be answered by using a ruler? (A) Which liquid makes bubbles that are the largest? (B) Which liquid makes bubbles that are the prettiest? (C) Which liquid makes bubbles that are the best smelling? (D) Which liquid makes bubbles that are the most interesting?
Example Output: liquids, bubbles., answered, using, ruler?

Example Input: Question: What effect does lack of food, disease, and predators have on a population? (A) They increase the mutations in a population. (B) They prevent a population from becoming extinct. (C) They increase the rate of population growth. (D) They can limit the growth of the population.?
Example Output: lack, predators

Example Input: Question: David was walking through his neighborhood and found a rock that had imprints of shells. What can you conclude about that area? (A) The area was once covered with trees. (B) The rock was carried there by the wind. (C) There had once been an ocean or river located there. (D) The area had a large amount of snow at one time.?
Example Output:
rock, imprints, shells., conclude, area?