[問題]Prep-T2-Q67

[sa_pot]

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

答案c
刪去a b後
c d e 該怎麼選呢？

[dibert8]

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

(A) (B) 不是句子
(D) (E) to 多餘; which = bridge 不會 show...

[lindababy]

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

(A) (B) 不是句子
(D) (E) to 多餘; which = bridge 不會 show...

當時做題就卡在a pair of
就覺得應該單數，但卻用了have，讓我遲遲選不下去
查了一下字典

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.

看起來有點像是紅色字體的用法，但又不完全是
因為這裡是a pair of
不知道有沒有高手可以指點一下
究竟是為什麼要用have呢?

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