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: What is the main function of a plant stem? (A) to collect pollen from other plants to make seeds (B) to absorb water and plant nutrients from the soil (C) to carry food and water to the rest of the plant (D) to make food for the plant?
Example Output: function, plant, stem?

Example Input: Question: Which of these organisms has cells with cell walls? (A) bluebird (B) A pine tree (C) A ladybug (D) A fox squirrel?
Example Output: organisms, cells, cell, walls?

Example Input: Question: A student places a sheet of black construction paper on her desk. What happens to most of the light that strikes the black construction paper? (A) The light is bent by the paper. (B) The light reflects off the paper. (C) The light is absorbed by the paper. (D) The light passes through the paper.?
Example Output:
happens, light, strikes, black, paper?