A particular environmental change causes the deaths of 25 individuals in a herd of 100 wild horses, and it kills 50 individuals in a herd of 200 horses. In this case, the growth of a wild horse population is most likely limited by . (Assume that the two herds are found in territories of equal size.)

a. the presence of another species that uses the same food resource
b. food supply
c. a predator
d. a density-independent factor
e. a density-dependent factor