Genetic drift is one mechanism of evolutionary change. Which of the following best describes an example of genetic drift?

A.
A small population of frogs, which used to be brown and green, is now all green due to chance.
B.
Gazelles, which are preyed on by cheetahs, have gotten faster and more agile over time.
C.
The genetic variation between two populations of gorillas decreases due to interbreeding.
D.
Over time, birds acquired stronger beaks by opening and eating tougher nuts and seeds.