Most energy analysts now agree that the costs of building and maintaining nuclear reactors are too high for nuclear power to likely prove cheaper than coal or oil in the long run.

(A) too high for nuclear power to likely
(B) high enough for nuclear power to be unlikely to
(C) high enough that it is unlikely nuclear power will
(D) so high that nuclear power is unlikely to
(E) so high as to be unlikely that nuclear power will

OA is D.

I had 2 issues with this question.
1. Is it correct to compare nuclear power with coal or oil?
2. Which of the 2 is a better usage? 'nuclear power is unlikely to prove cheaper...' or 'nuclear power is unlikely to be proved cheaper...'?

Please help.