一道难题gwd-11-3 求助

For protection from thesummer sun,the Mojave lived in open-sided,flat-topped dwellings known as shades,each a roof of poles and arrowweed supported by posts set in a rectangle.

A.  each a roof of poles and arrowweed

B.  each a roof of poles and arrowweed that are being

C.  with each being a roof of poles and arrowweed

D.  with roofs of poles and arrowweed to be

E.  with roofs of poles and arrowweed that are

答案选A，想不通，我选了E，题目意思都不太明白，请各位大侠帮忙看看

refer to：http://forum.chasedream.com/dispbbs.asp?BoardID=23&ID=55193；

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=65908&page=1；