Would the following examples of reproductive isolation be considered a prezygotic or postzygotic mechanism?
A. Horses and donkeys can interbreed to produce mules, but the mules are infertile.
B. Three species of the orchid genus Dendrobium produce flowers 8 days, 9 days, and 11 days after a rainstorm. The flowers remain open for 1 day.
C. Two species of fish release sperm and eggs into seawater at the same time, but the sperm of one species do not fertilize the eggs of the other species.
D. Two tree frogs, Hyla chrysoscelis (diploid) and H. versicolor (tetraploid), can produce viable offspring, but the offspring are sterile.