If oil leaks from a tank at a rate of $r(t)$ gallons per minute at time $t$, what does $\int_{0}^{\,120} {r(t)}\,dt$ represent?

A. The average rate at which oil leaked from the tank during the first two hours.
B. The rate at which the oil leaks out after two hours have passed.
C. The number of gallons remaining in the tank after the first two hours have passed.
D. The number of gallons of oil that leaked from the tank in the first two hours.