一道完全没有sense的题

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

Answer: a

完全没有sense，求达人解释下

each强调型独立主格结构为，主句+复数名词结尾，each + -ing短语/-ed短语/介词短语/形容词短语/名词短语

这道题太猛了，呵呵，叫人血脉奋张的说，这样就知道只能在AB之间选择了，B没什么可说的that are being这用法看着就够怪的了，完全可以用-ed分词做后置定语。我一开始以为楼主MM的arrowweed写错了，因为字典里面怎么也找不到，google了一下才知道是只用灌木an upright shrub often growing 7-9 feet tall with several long, straight, stiff stems per plant.汗死了