GMAT Sentence Correction (SC) | Re: By pressing a tiny amount of nitrogen between two diamonds

Hi,

It is true that we generally use “not only” pairing it with “but also” to denote two aspects of the same thing. However, the use of “not only… but…” is not incorrect (but that we encounter in rare occasions).

We use “not only A but also Y” or “not only X but Y”, what we must pay attention to is that the entities following both the markers (X and Y) must be logically as well as grammatically parallel.

In this sentence, only choice C adheres to this rule of parallelism: