请教GWDTT10-14

Q14:

The single-family house constructed by the Yana, a Native American people who lived in what is now northern California, was conical in shape, its framework of poles overlaid with slabs of bark, either cedar or pine, and banked with dirt to a height of three to four feet.

A.    banked with dirt to a height of

B.    banked with dirt as high as that of

C.    banked them with dirt to a height of

4. was banked with dirt as high as

5. was banked with dirt as high as that of

我查了下帖子,说a是独立主格结构,banked是adj, 如果这样,独立主格主语在哪里呢?

谢谢!