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
was banked with dirt as high as
was banked with dirt as high as that of
The reference answer is D, but mine is A. Can anyone help to clarify this? Thanks! | 
