Usual

The new sports car is heavier than usual for a performance automobile, but it is exceptional because of its high-powered engine.

A) usual
B) it is usual
C) one usually is
D) is usual
E) has been usual

My first answer was A (how silly of me:) )

This is what mba wrote

(D) it is,

(A) makes an improper comparison.
(B) "it" has no referent.
(C) "one" has no referent
(E) has a tense problem.