What type of reproductive barrier applies to each of these scenarios?

a. Water buffalo and cattle can mate, but the embryos die early in development.

b. Scientists try to mate two species of dragonfly that inhabit the same pond at the same time of day. However, females never allow males of the other species to mate with them. 

c. One species of reed warbler is active in the upper parts of the canopy, while another species of reed warbler is active in the lower canopy. Both species are active during the day.

d. Scientists mate two parrots from different populations to see if speciation has occurred. The parrots mate over and over again, but the male’s sperm never fertilizes the female’s egg.