For protection from the summer 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.

1. each a roof of poles and arrowweed
2. each a roof of poles and arrowweed that are being
3. with each being a roof of poles and arrowweed
4. with roofs of poles and arrowweed to be
5. with roofs of poles and arrowweed that are

后面的定语从句修饰shades,是描述shades的样子，屋顶是a roof of poles,身子是arrowweed supported by posts set in a rectangle

b.each a roof of poles and arrowweed that are being（多余，不简洁）

c.with each being(being+n.错） a roof of poles and arrowweed

d.with roofs（把roof和arrowweed用同样的修饰，改变原意，错） of poles and arrowweed to be

e.with roofs（同d） of poles and arrowweed that are

这样就很容易得出答案是a了