In the Figure Below (Left), Two Populations of One Species

In the figure below (left) , two populations of one species of fruit flies are raised for several generations on different nutrients (starch and maltose) . On the right, the members of the two populations were reintroduced and the mating frequencies of flies from the two different populations were recorded.
Which of the following is true?

A) After only a few generations, these two populations have undergone speciation since they no longer mate with each other at high levels.
B) The two populations are in the early stages of reproductive isolation since male flies are more likely to mate with female flies that were raised on the same medium.
C) Fruit flies grown on starch for several generations are no longer capable of consuming maltose.
D) The original population of fruit flies actually consisted of two different species of fruit flies. When hybridization occurred between the two populations, only sterile offspring were produced.