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 student has a ball of clay that sinks when placed in a pan of water. Which property should he change to make the clay float? (A) color (B) texture (C) mass (D) shape?
Example Output: clay, sinks, water., property, change, float?

Example Input: Question: A light bulb that is lit with a battery would ___. (A) produce kinetic energy (B) transform mechanical energy into electrical energy (C) give off light and heat (D) give off light but remain cool?
Example Output: light, bulb, lit, battery

Example Input: Question: Feathers, wings, and the hollow bones of birds are examples of (1) adaptations for flight (2) responses to stimuli (3) unnecessary body parts (4) reproductive structures?
Example Output:
hollow, bones, birds