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.

[Q]: Question: Campers shouted toward a canyon wall. What characteristic of sound best explains why the campers heard an echo? (A) Sound waves traveled slowly down the canyon. (B) Sound waves were absorbed by the canyon wall. (C) Sound waves reflected off the canyon wall. (D) Sound waves were refracted around the canyon.?
[A]: wall., characteristic, sound, explains, campers, echo?


[Q]: Question: The most important safety rule is (A) follow all instructions. (B) heat all substances before smelling. (C) clean all equipment. (D) mix all chemicals before starting an experiment.?
[A]: important, safety, rule


[Q]: Question: Valerie pumps her super squirt gun and squirts her friend. At which point is the amount of potential energy greatest? (A) As the water almost reaches her friend (B) When the squirt gun is being pumped (C) Just before she squirts her friend (D) As the water first leaves the squirt gun?
[A]:
pumps, squirt, gun, squirts, potential, energy, greatest?