Please choose the correct answer to the question: 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?

Answer1: the accumulation of the sound waves Answer2: the diffraction of the sound waves Answer3: the refraction of the sound waves Answer4: the reflection of the sound waves

Answer format: answer1/answer2/answer3/answer4
Answer: answer4