Here's the
official explanation provided by the GMAC for this question:
What
are prohibitive ? For the sake of subject-verb agreement, the plural noun
costs must be used rather than the singular
cost :
the costs… are prohibitive The phrase
associated with is required to complete the parallel construction
costs associated with upgrading… and with the development of… . Option A: Singular
cost does not agree with the plural verb
are Option B: Correct The plural noun
costs agrees with the plural verb
are ; using
costs associated with means that
with upgrading is parallel to
with the development .
Option C: From upgrading is not parallel to
with the development of Option D: Singular
cost does not agree with the plural verb
are ;
of upgrading is not parallel to
with the development of Option E: Of upgrading is not parallel to
with the development of The correct answer is B.
Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.