http://www.manhattangmat.com/forums/by-pressing-a-tiny-amount-of-nitrogen-between-two-diamonds-t1459.html 那个上面是这样说的: not really. what's actually happening is that there's a second correctidiom, "not only ... but ...", which you just didn't happen to know.
rememberthat idiomatic construction is entirely up to the preferences of thegmat staff; there will always be one or two constructions that you justdidn't know. occasionally, the gmat even surprises or frustrates me with its choices of correct and incorrect idioms, and i've seen lots and lots and lots of formal written english.