According to a government study, the lush swamps and marshs of the Mississippi delta, among the finest wetlands in the world, are vanishing at a rate of 39 square miles a year, as fast as two and a half times the rate that was previously thought.
(A) as fast as two and a half times the rate that was
(B) two and a half times as fast as it had been
(C) two and a half times faster than
(D) a rate two and a half times as fast as
(E) which is a rate two and a half times faster than had been
答案是C
想請問D哪裡不好?