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.
Answer: plucking
Plucking is a type of glacier erosion.Plucking occurs when the ice around the broken rocks, gets displaced with the force exerted by the retreating glacier. The glacier ice plucks the freezed rocks away along with it during glacier erosion.
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