442.                  It is characteristic of the Metropolitan Museum of Art, as of virtually every great American museum, the taste of local collectors has played at least as large a part in the formation of their collections as has the judgments of the art historian. 

(A) of virtually every great American museum, the taste of local collectors has played at least as large a part in the formation of their collections as has

(B) of virtually every great American museum, that the taste of local collectors has played at least as large a part in the formation of their collections as has

(C) it is of virtually every great American museum, that the taste of local collectors has played at least as large a part in the formation of its collections as have

(D) it is of virtually every great American museum, that the taste of local collectors have played at least as large a part in the formation of its collections as have（C）

(E) it is of virtually every great American museum, the taste of local collectors has played at least as large a part in the formation of its collections as has

请问，C中的that  没有搞懂到底起什么作用的？ 请过路的XDJ看看？ 谢谢

后面怎么用 have 了