An Insect Population Lives Along the Edge of a North-South

An insect population lives along the edge of a north-south mountain range.The populations from the east and west slope eventually join in a low northern pass and interbreed,producing fertile offspring; but they do not circle around the southern edge because of a desert barrier.When glaciers move southward,the populations are pushed south of the northern pass and are isolated.While isolated,the two populations develop enough differences over time that when the glaciers retreat north and the insects again share the same pass,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.