Rocky Mountain juniper (Juniperus scopulorum) and one-seeded juniper (J. monosperma) have overlapping ranges. Pollen grains (which contain sperm cells) from one species are unable to germinate and make pollen tubes on female ovules (which contain egg cells) of the other species These two juniper species are kept separate by:______
a. temporal isolation
b. habitat isolation
c. behavioral isolation
d. gametic isolation