Task: Question Answering
Input (English): Some students were on a tour exploring the inside of a cave. While the students were in the cave, they realized they heard an echo every time that they spoke. Which most likely caused the echo to occur?  
 (A) the accumulation of the sound waves (B) the diffraction of the sound waves (C) the refraction of the sound waves (D) the reflection of the sound waves
Output (English): D