请帮我解释一下这道题。谢谢。

txsnowsea

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
   选A
   

wudiyeah

each型独立主格，指代其前复数名词