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.
5.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. (A) each a roof of poles and arrowweed (B) each a roof of poles and arrowweed that are being (C) with each being a roof of poles and arrowweed (D) with roofs of poles and arrowweed to be (E) with roofs of poles and arrowweed that are 答案A, 为什么会用each a roof, 而不用each roof 呢?