单选题根据下面的内容，回答题.

Which of the following may be a density-independent factor that affects animal population?

A．Famine.
B．Tornadoes.
C．The number of predators.
D．The variety of food supply.

A．explain the population growth of organisms with short life cycles.
B．re-produce the results of the study of Christian and Davis.
C．show how many mechanisms that manage regulation there are.
D．make sufficiently accurate predictions about the effects of crowding.

A．As the number of foxes in Minnesota decrease, the growth rate of this population of foxes begins to in-crease.
B．As the number of woodpeckers in Vermont decreases, the growth rate of this population of woodpeckers also begins to decrease.
C．After the number of prairie dogs in Oklahoma increases, the number of predators of these dogs also begins to increase.
D．After the number of beavers in Tennessee decreases, the number of predators of these beavers begins to increase.