GWD-13-29

Q29:

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

请问：A是什么语法现象，独立主格？

怎么解释？each a roof of poles and arrowweed