Sentence Correction

Hi All,

In the following question OG-10, Q4, OA = E. For kicking out C, OG says...

"It must refer to something (unlike in phrases such as "it is clear that...."), yet it does not plausibly refer to anything.

Can someone please tell what does OG mean to say here ? Why cannot "it" act as simple place holder as Manhattan SC guide explains....for e.g. there is no referent for it in "It is raining outside"

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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