Sentence Correction

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

OA: 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.

as for D, 'that' is singular and it does not agree with 'speeds'... to be grammatically correct is should be 'at speeds much higher than those currently possible'.

for me it was a toss-up between A and E (B: wordy, C: irrelevant 'as', D: see above). I wasn't sure between:

(A) ...than (it) is currently possible
(E) ...than (the speeds which) are currently possible

However, idiomatically you say 'at speeds' and not 'with speeds' - so E wins.