The difference between the two students that we know of is that they used a different meter stick. So the difference would be best attributed to the different tools they used.
Note, this is quite a huge difference. It's likely that one of the sticks, or both, are quite defect.
D.8 should be the correct answer
Yes, waves can carry energy through
To reduce the likelihood of human error, to gather more evidence, and to make his numbers more exact.
