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.