**Geographic isolation caused the separation of rainforest frog populations into a population in the north and a population in the south. The separated populations later reconnected because the climate got wetter
and warmer, causing the rainforest to expand. When males from the north mated with females from the
south, the offspring failed to develop past the tadpole stage. When males from the south mated with
females from the north, the offspring developed more slowly than the offspring of pairs of northern frogs.
These data support that which event occurred while the two populations of frogs were separated?
a. The two populations developed into new species.
b. The two populations mated with other species of frogs.
C. The two populations began a new method of reproduction.
d. The two populations had fewer offspring than before the separation.