A geologist found a sediment deposit at the open end of a valley. After studying the deposit, the geologist concludes the sediment was deposited by the front end of a glacier. There is a glacier in the valley, but it has melted back 2.5 km up the valley (see diagram below).

How can the geologist estimate how long ago the glacier was in the lower part of the valley?
A.
Watch the glacier melt for a few hours, and see how long it takes for the distance between the glacier and the sediment deposit to double.
B.
There is no way to estimate this—the geologist must make a random guess.
C.
Divide the distance the glacier has melted back by the current rate at which it is melting back.
D.
Measure how long it takes for the rest of the glacier to melt, which will be the same amount of time.