Juanita learns that gray tree frogs and green tree frogs both live in the southern United States. The gray tree frogs live in forests and blend in with the bark on the trees. The green tree frogs blend in with the tall reeds and grasses of the wetland areas. Natural selection helps the gray tree frogs in the forests and the green tree frogs in wetlands.

Natural selection is affected by which three factors?
A. reproduction
B. evolution
C. variation
D. overproduction
E. mechanism
F. competition