@skm

The non-underlined part clearly says- how their idea would work? What does that ‘their’ refer to? It is not going to be bridges or structures or any other inanimate noun for that matter. It has to be only the pair of architects. When the text itself calls it plural, how can we doubt it? That is the reason; I asked to read the entire text for some veritable clues. So going along the context is the right track
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Now on to sub-verb agreement: For the argument that the subject is the singular a pair, May I ask to read the entire text and look for further clues( especially in the non-underlined part) as to whether the subject is singular or plural. This kind of error is easy meat for GMAT, for many jumps to wrong conclusions with just a casual glance
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Before answering I just want to point out that someone must have deleted the original question, because my post is the answer, not the question!

shaileshmishra, you may find this helpful:

Usage Note: The noun pair can be followed by a singular or plural verb. The singular is always used when pair denotes the set taken as a single entity: This pair of shoes is on sale. A plural verb is used when the members are considered as individuals: The pair are working more harmoniously now. After a number other than one, pair itself can be either singular or plural, but the plural is now more common: She bought six pairs (or pair) of stockings.

source :pair

This should carify, let me know

Before answering I just want to point out that someone must have deleted the original question, because my post is the answer, not the question!

shaileshmishra, you may find this helpful:

Usage Note: The noun pair can be followed by a singular or plural verb. The singular is always used when pair denotes the set taken as a single entity: This pair of shoes is on sale. A plural verb is used when the members are considered as individuals: The pair are working more harmoniously now. After a number other than one, pair itself can be either singular or plural, but the plural is now more common: She bought six pairs (or pair) of stockings.

source :pair

hi Zarrolou and daagh,
I think pair goes singular only when we talk about items that consist of both things. What I'm trying to say is that if we say a pair of socks or shoes or lenses then both things form whatever we're talking about. A pair of architects doesn't form any such thing.

hope i am correct...
please suggest

SKM
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ya zarrolou.....it seems gud....let me encounter some more problem basing this....
thanks anyways...
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I only said context decides the number in certain cases. Zarrolou has gone a step ahead and elaborated on the plausible contexts. His explanation is more wholesome and focused .
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hi rrkan,

the original given sentence is as follows:
A pair of architects in Britain, who say that giant arches, bridges, and walls made of artificial bone could be easier to design and [u]build than conventional structures, and already designed a number of structures, including a bridge, for showing how their idea would work.
the coloured part starting with "who say .....than conventional structures"...actually this whole part is acting as a modifier and it is modifying a pair of architects.
now good thing with modifier is that if you remove the modifier from the sentence then also sentence must make sense.
now lets remove this modifier and hence resulting sentence will become like this:
A pair of architects in Britain and already designed a number of structures, including a bridge, for showing how their idea would work.
now reading the sentence do you think does this makes sense.
the use of parallelism marker AND conveys that both side of the and should be parallel but here
A pair of architects in Britain
and
already designed a number of structures
is not parallel ALSO THERE IS NO MAIN VERB TOO IN THIS SENTENCE.
hence use of and is wrong. in the same way option B is also wrong.
again now in option D after removing the modifier the resulting sentence becomes:
A pair of architects in Britain already designed a number of structures, including a bridge,which shows how their idea would work.
now if you read the above sentence then you will find that there is no verb in the sentence for the subject "a pair of architect"
and the use of HAVE before already rectifies this error which is done in option C.
so in this case use of already is not wrong but rather highlighted already indicates that a verb is required before already.
SKM
KUDOS IF IT HELPED.
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Great Q. Helps to develop a better understanding of use of 'pair' as plural vs. singular.

A pair of architects in Britain, who say that giant arches, bridges, and walls made of artificial bone could be easier to design and build than conventional structures, and already designed a number of structures, including a bridge, for showing how their idea would work.

(A) build than conventional structures, and already designed a number of structures, including a bridge, for showing
(B) build than conventional structures, and they have already designed a number of structures, which includes a bridge, to show
(C) build than conventional structures, have already designed a number of structures, including a bridge, to show
(D) also to build than conventional structures, already designed a number of structures, including a bridge, which shows
(E) to build than with conventional structures, have already designed a number of structures, including a bridge, which shows

look at choice D
" a bridge, which shows' is a bridge we already know that this bridge shows. this meaning is not fit in the context of the sentence. D is gone
if we have " a bridge that shows", we have to understand that there are many bridges and we mention the bridge that shows... . also this meaning is not fit to the sentence.

difference between " that shows " and " which shows" is hard but gmat never test us this difference.