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.

Input: Consider Input: Question: Natural processes shape the surface of Earth. Most canyons are formed by (A) ice (B) plants (C) steady winds (D) moving water?

Output: canyons, formed


Input: Consider Input: Question: What is the main source of heat energy on Earth? (A) fire (B) volcanoes (C) the sun (D) electricity?

Output: main, source, heat, energy, Earth?


Input: Consider Input: Question: Peach trees have sweet-smelling blossoms and produce rich fruit. What is the main purpose of the flowers of a peach tree? (A) to attract bees for pollination (B) to create flower arrangements (C) to protect the tree from disease (D) to feed migratory birds?


Output: purpose, flowers, peach, tree?