I guess there is some ruling that the vehicle must be able to maintain a given altitude (over MSL) without any conditions to the terrain below. That would exclude hovercraft, as they are always some given distance from the ground, not a given altitude over MSL.
But maybe nobody has ever asked this question before and there is no jurisdiction about that. I'm just speculating here
WARNING! The post above should be taken with a grain of salt! Furthermore, it may be slightly biased towards A.