问：GWD31套第1套Q3，thanks！

1of 31 Q3:

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

划线部分是什么意思啊，怎么还有一个each，搞不明白，战友们帮忙看看吧，十分感谢！

each 指的是 each shade，後面接的 a roof of poles and arrowwood (題目打錯字母了)supported by posts set in a rectangle 整個片語是形容each shade，意思大致是"由支柱撐起來的，使用竿子和arrowwood(某種木頭，但我不知道中文是什麼)、呈現矩形的屋頂"。

不知道這麼解釋，清不清楚...