Two Populations of Insects Interbreed, Producing Fertile Offspring

Two populations of insects interbreed, producing fertile offspring. Due to glacial activity, the two populations become isolated. Many years later, the populations are reunited but they no longer mate at the same time, nor can they produce fertile offspring. These insects

A) began as one species and therefore remain one species.
B) were originally two species and remain two species.
C) were originally two species but are now one species.
D) were originally one species but are now two species.
E) The number of species cannot be determined from the information given.