Why Can't Prokaryotic Species Be Defined as a Group of Similar

Why can't prokaryotic species be defined as a group of similar organisms that could sexually reproduce together?

A) The mating rituals of bacteria have not been studied enough.
B) Bacteria are all too different to be considered similar.
C) We can't see them in enough detail to tell how similar they really are yet.
D) Bacteria reproduce asexually.
E) They can be.