Which of these scenarios is an example of natural selection?

A.) Farmers crossbreed two types of cows to produce a breed of cow that has high-quality beef and can tolerate a hot climate.
B.) Dogs of the same breed and having a desirable coat color are bred with one another to produce more puppies with that same coat color.
C.) Wolves with thick coats survive a cold winter more than wolves with thin coats. Thick-coated wolves have more offspring the following spring.
D.)People kill rattlesnakes that rattle when approached by humans, but people do not kill rattlesnakes that remain silent. The quiet snakes survive and breed.