GMAT Sentence Correction (SC) | Re: A pair of architects in Britain, who say that

A pair of architects in Britain, who say that giant arches, bridges, and walls made of artificial bone could be easier to design andbuild 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,