In 1987, 18 Black-Footed Ferrets, the Last Known Individuals of This

In 1987, 18 black-footed ferrets, the last known individuals of this species, were captured and brought into a captive breeding program in Wyoming. In 1989, the total ferret population, still in captivity, was 120 animals. These 120 animals in 1989 represented:

A) N, the total population size.
B) $\Delta$ N, the change in population size from 1987 to 1989.
C) $\Delta$ N/ $\Delta$ t, the rate of change in population size.
D) r, the per capita change in population size.
E) r, the exponential rate of growth of the population.