Not only do deep-sea divers risk nitrogen narcosis, often called "raptures of the deep", if they descend below 200 feet, but they also fall prey to decompression sickness, commonly known as the "bends", if they ascend too quickly.

a] Not only do deep-sea divers risk nitrogen narcosis, often called "raptures of the deep", if they descend below 200 feet, but
b] Deep-sea divers risk nitrogen narcosis, often called "raptures of the deep", if they descend below 200 feet, but
c] Nitrogen narcosis, often called "raptures of the deep", is risked by deep-sea divers if they descend below 200 feet, in addition
d] Descending below 200 feet causes deep-sea divers to risk nitrogen narcosis, often called "raptures of the deep" and
e] Not only does a deep-sea diver risk nitrogen narcosis, often called "raptures of the deep", if they descend below 200 feet, but

OA: A

Can instructor please expain..

Not only X, But Y

Here there is a pronoun after But .

My question is if we follow Not only X, but Y rule then how can the right answer be A

source Kaplan GMAT

I am not an instructor but I could help.

The fundamental usage is "not only ...but also.."
not X but Y and not only X but also Y are also acceptable idioms.

>>>
I am not an instructor but I could help.

The fundamental usage is "not only ...but also.."
not X but Y and not only X but also Y are also acceptable idioms.
<<<<


My question is how does/not the pronoun "they" change the meaning and if it can be considered a correct idiom.


thanks

Substitute deep-sea divers for X and they for Y.

The sentence would read :

Not only do deep-sea divers .........But they(referring to deep-sea divers) also.....

This is correct in terms of idiomatic usage and the intended meaning remains intact.
Try to analyze this sentence.

Not only did the swimmer win the 100 m free style event but he also got selected for the olympic games.

Here the pronoun(he) refers to the swimmer. So this follows not only X but (pronoun) also... which is an acceptable usage.

Another Example:

Not only do students of the high school study well, but they also excel in sports.---->Here they refers to students and the usage looks fine.

Hence even though there is a pronoun usage within the idiom its usage is proper.

Hope I could help!