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: Measurements of carbon dioxide concentrations in the air taken over the past decade have shown a (A) leveling off. (B) slow increase. (C) steady decline. (D) dramatic increase.?
Example Output: Measurements, carbon, dioxide, concentrations, air, past, decade

Example Input: Question: Which will most likely cause a decrease in predator populations? (A) an increase in prey populations (B) a decrease in prey populations (C) a decrease in decomposers (D) an increase in producers?
Example Output: cause, decrease, predator, populations?

Example Input: Question: Earth is composed of four layers. Many scientists believe that as Earth cooled, the denser materials sank to the center and the less dense materials rose to the top. The least dense layer is the (A) crust (B) mantle (C) outer core (D) inner core?
Example Output:
Earth, Earth, least, dense, layer