The Faster a Cheetah Can Run, the More Likely It

The faster a cheetah can run, the more likely it is to capture its prey. Cheetahs with longer legs are able to run faster than those with shorter legs. Although the existing cheetah population shows a variety of leg lengths, cheetah legs are not increasing in length from one generation to the next. Which of the following is the most likely explanation?

A) Cheetah populations no longer contain the alleles needed to grow longer legs.
B) Cheetahs with long legs are reproductively isolated from cheetahs with short legs.
C) Natural selection does not act on the genes involved in determining leg length.
D) The longer the length of a cheetah leg, the more likely the leg bone is to break.