og12 4

none123

Of all the vast tides of migration that have swept through history, maybe none is more concentrated as the wave that brought 12 million immigrants onto American shores in little more than three decades.

(A) maybe none is more concentrated as

(B) it may be that none is more concentrated as

(C) perhaps it is none that is more concentrated than

(D) maybe it is none that was more concentrated than   
(E) perhaps none was more concentrated than
答案倒是没有问题，但是og里面解释说c和d选项里面的it必须有指代，但是cd里面没有指代。
我比较疑惑这个强调句型的it不就是应该没有指代的吗？把 it is that去掉之后不还是一个完整的句子？
求解释！

victor198506

none本身就是代词啊