Sentence Correction

Dear friends,

I need your help clarify two questions here,

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

i have two questions here., one is tense in D, the other is "with conventional structures" in E, please help clarify.

firstly, tense in D,
I cross off D because there is a word "already" in D, as i know, you should use past perfect when you see a word such as "already", but i am not sure whether it is the same rule in GMAT

senondly, "with conventional structures",
honestly speaking, i picked up E, because
what i read is preceding concretes such as "giant arches, bridges, and walls " , while, "conventional structures" is abstract, which is upper level noun than "giant arches, bridges, and walls ", i don't think concretes can be compared directly to it's upper level noun, that's why i picked up E.
Please help.


thanks a lot, have a nice day.

>_~