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 of the following is the best observation of the refraction of light? (A) Looking at oneself in a mirror (B) Looking at a shadow of an object (C) Viewing an object through an open window (D) Viewing an object partially in water?
Example Output: observation, refraction, light?

Example Input: Question: Timber companies can best manage the number of trees in forests by (A) converting deforested areas into malls. (B) planting trees to replace what they cut down. (C) adding pesticides to the soil around the trees. (D) removing the tree stumps when trees are cut down.?
Example Output: Timber, companies, manage, number, trees, forests

Example Input: Question: A cat jumps when startled by a loud noise. This is an example of an organism (1) sensing and responding to its external environment (2) sensing and responding to its internal environment (3) changing and controlling its external environment (4) changing and controlling its internal environment?
Example Output:
cat, jumps, startled, noise., example