Scientists Investigating Speciation in Fruit Flies Divided a Single Population

Scientists investigating speciation in fruit flies divided a single population of flies into separate populations that each lived in their own cage on one of two food sources: maltose-based food or starch-based food. After several generations, the flies' mate preference was tested, and it was found that flies strongly preferred to mate with other flies that were raised on the same food source. This results suggests that

A) selection on food source indirectly led to the evolution of reproductive isolating mechanisms.
B) selection on reproductive isolating mechanisms indirectly led to the evolution of different food preferences.
C) reproductive isolating mechanisms evolved primarily through genetic drift.
D) different food preferences evolved primarily through genetic drift.