The following options are all reproductive barriers.
Which one is an example of a prezygotic barrier in which the two different species do mate?

a. Two crayfish species cannot interbreed because their genitalia do not engage properly with each other.
b. Hybrid mules produced by mating donkeys and horses are sterile.
c. One bird species lives in wetlands, the other in dry forests; hence, they do not encounter each other.
d. Two dragonfly species regularly mate, but their gametes are incompatible.