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: A squid is an animal that lives in the ocean. It pumps a stream of water out of its body, causing it to move rapidly. Which of the following mechanical systems moves in a way most similar to the squid? (A) a bus (B) a helicopter (C) a rocket (D) a train?
Example Output: squid, pumps, water, mechanical, systems, similar

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: A large tree was cut down with a saw. What is the best way to estimate the age of this tree? (A) measure the height of the tree (B) measure the thickness of the bark (C) count the number of rings in the trunk (D) count the branches growing from the trunk?
Example Output:
tree, cut, saw., best, estimate, age