Fruit flies all look more or less alike. If you have a male and female fruit fly, how could you test the hypothesis that they're the same species?

A) Determine the sequence of several of their genes.
B) Examine them closely under a microscope, and compare what you see to a published description of fruit flies.
C) See if they'll mate when they're put together.
D) See if they mate and if their offspring can also mate.