24套GWD的第六套的第二个语法题

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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

  请问为什么要选A啊  其它的选项为什么不选哦

从容自适

自己先顶下  牛牛们 现身帮帮想想吧!