I believe B to be right here
I'm not sure what the exact rule here is but I'll try to back it up with examples.
Ex: A mercedes is more expensive than usual for a car --> wrong
A mercedes is more expensive than is usual for a car --> sounds better
Warren Buffet is richer than usual for a human being --> wrong
Warren Buffet is richer than is usual for a human being --> sounds better
I believe the difference between the 2 is that the former is equivalent to normal and you usually intends "compared to the subject's self"
Ex: He is nicer than usual --> ok
The latter "is usual" is used to compare the subject, being part of a subgroup, to the overall group as shown in my examples.