As Stomme said it's an empty element and to my mind a span would be just as incorrect because a span infers that some inline mark up is about to appear and of course there isn't any.
If you look at many of the round corner techniques they are all using <b> to save thousands of characters per page rather than using spans. As I said above it's really a misues of the element but does make practical sense in some cases. (We could really do with a new element like <x> element that means nothing and should be ignored by all except css.)
Anyway back to the quiz and later on I will float in and drop you all a few more hints