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
答案是c， 我想问的是： A pair of 。。。谓语为什么这里用了have already...， 不时应该用has才对吗？ 我理解这个句子的结构是： A pair of architects in Britain, who...., have already designed....
难道我的理解有问题吗？请指教。非常感谢！ | 
