What Is the Biological Species Concept, and What Is One

What is the Biological Species Concept, and what is one problem with it?

A) It defines a species as being genetically isolated, but scientists can't determine that.
B) It defines a species as being unable to interbreed with any other species, but all species can interbreed if given the opportunity, so it doesn't apply to the real world.
C) It defines a living species as mostly able to interbreed but doesn't work with fossils.
D) It defines a species as being unable to interbreed with every other species, but a few species can interbreed to some degree so it's not clear-cut.
E) It defines a species as being genetically isolated from every other species, but some organisms, like bacteria, are all genetically isolated all the time, so it doesn't work for them.