请教prep上一道语法题

问题同上。。。。

sorry贴图不成，再来！

The Anasazi settlements at Chaco Canyon were built on a spectacular scale with more than 75 carefully engineered structures, of up to 600 rooms each,were connected by a complex regional system of roads.

A.with more than 75 carefully engineered structures, of up to 600 rooms each,were

B.with more than 75 carefully engineered structures, of up to 600 rooms each,

C.of more than 75 carefully engineered structures of up to 600 rooms ,each that had been

D.of more than 75 carefully engineered structures of up to 600 rooms and with each

E.of more than 75 carefully engineered structures of up to 600 rooms each had been

选对了，但是说不清为什么。就觉得CDE一团团在一起不知道在说什么，A又多出来一个were，也排除，留下B。

帮忙顶，请问each指代的是什么？

Each settlement at Chaco Canyon was built of up to 600 rooms？

这里的of，是build of还是each of呢？