By the mid-seventeenth century, Amsterdam had built a new town hall so large that only St. Peter’s in Rome, the Escorial in Spain, and the Palazza Ducale in Venice could rival it for scale or magnificence.
(A) could rival it for
(B) were the rivals of it in their
(C) were its rival as to
(D) could be its rivals in their
(E) were rivaling its

Can you please explain why the answer is A and not D or E? Thanks.

Tough one! Let's first cut this down to the core:

By <a time in the past>, A. had built an X so large that only A, B, and C...

First, there's an idiom. A, B, and C rival X for <some attributes>. If you want to use rival in verb form, you just put the X right after (or a pronoun referring to X, as in this case) and you use the preposition "for" to introduce what the attributes are.

E tries to use the verb form. But "were rivaling its" sounds like A, B, and C are rivaling "scale and magnificence." That doesn't make sense - A, B, and C are buildings and they should be rivaling some other building or buildings. Think of this as a comparison almost - you actually have to mention the two things you're comparing on either side of the comparison language.

So E is gone.

D changes the form of the word rival - now it's a noun. A, B, and C could be X's rivals in their scale and magnificence. Again, idiom issue. You could say that A, B, and C are the primary rivals of X, but you wouldn't present the attributes by which they rival X with the preposition "in." You'd say something like "A, B, and C are the primary rivals of X based upon <some attributes>." (Note: there are other words you could use besides "based upon" - eg, "due to." But not "in.")

Also, introducing the word "be" after "could" now makes it sound like someone's speculating - "hmmm, could A, B, and C rival X?" But that's not the original meaning - the original meaning is essentially saying that these guys (A, B, and C) really are the only possible rivals, not that they might be but the author's not sure.

_________________
Stacey Koprince
Instructor
Director of Online Community
ManhattanGMAT

about Rival sb in/for some acpect:
i have search in Longman, there are some sentence can support Rival sb in XX, which is also listed as a idiom.
1#The weathermen said the storm had rivalled summer hurricanes in its intensity.
2#The prince built a vast palace, rivalling Versailles in size and opulence.

Unfortunately, i can't find material that can support Rival sb for XX, except when Rival is used as Noun.
Stephen is Ron's rival for the job---Stenpen and Ron both need job, so we use 'for'.

i don't know what's wrong with D, which is really like 1# above. although i try several dictionnary, i can't find the usage of "rival [Vt] for". SO:
1# is A really better than--or just shorter than-- D?
2# are there some dictionaries that deserve recommendation for listing lots of idiom tested by GMAT?

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if A is the right key, there could be only one explanation:
X rival Y in(for) Z IS DIFFERENT FROM X is a rival of Y in(for) Z.
Ron, may be some intructors can be your rivals in teaching GMAT, but no one can rival you. Haha!

_________________
stephen