About adverbial clause of comparison

I have a question about a GMAT sentence correction problem,

According to analysts, an alliance between three major personal computer companies and most of the nation's largest local telephone companies would enable customers to receive Internet data over regular telephone lines with speeds much higher than is currently possible.
A. with speeds much higher than is
B. with speeds that are much higher than are
C. at much higher speeds as are
D. at much higher speeds than that
E. at speeds much higher than are

The official answer is E

In this problem, why is D wrong?
In my opinion 'currently possible' could be a post-positive adjective that modifies 'that'. I really appreciate your help.