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: Which is a nonrenewable resource? (A) oil (B) trees (C) solar energy (D) food crops?
Example Output: nonrenewable, resource?

Example Input: Question: The items listed below were found in a science classroom. * a heart made of plastic with many of its parts labeled * clay formed to look like Earth and other planets * a giant plastic plant cell with removable cell parts These items are all examples of (1) models (2) experiments (3) variables (4) controls?
Example Output: heart, plastic, parts, labeled, clay, Earth, giant, plastic, plant, cell, examples

Example Input: Question: Jake's cats eat food from a can. The cats come running into the kitchen when they hear the electric can opener. Now, each time Jake's mother opens a can of corn, the cats come running. This is an example of ___. (A) learned behavior (B) correct behavior (C) instinct (D) migration?
Example Output:
cats, food, can., hear, can, opener., opens, can, cats, running., example