Task: Question Understanding
Input (English): 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.?
Output (English): wall., characteristic, sound, explains, campers, echo?