Fruit Fly Species All Look Similar to One Another

Fruit fly species all look similar to one another. If you have a male and a female fruit fly, how can you prove that they are the same species, according to the biological species concept?

A) If they mate successfully and their offspring are fertile, they are the same species.
B) Examine them closely with a low- power microscope, comparing their physical characteristics to published species key lists of characteristics.
C) Determine the base sequence of the DNA of their chromosomes.
D) If they mate when they are put together, they are the same species.
E) If they can asexually reproduce and their offspring can also successfully asexually reproduce, they are the same species.