not only, but also里的also在gmat题中是否可以省略

我知道在口语里和在普通英语里not only, but also里的but是可以省略的    但在gmat的语法题里是否也是可以省略的呢   谢谢

also可以省略

By pressing a tiny amount of nitrogen between two diamonds to a pressure of 25 million pounds per square inch, scientists not only were able to transform the gas into a solid, but they also created a semiconductor similar to silicon.

(A) not only were able to transform the gas into a solid, but they also created

(B) not only were able to transform the gas into a solid but also creating

(C) were able not only to transform the gas into a solid but to create

(D) were able not only to transform the gas into a solid but also creating

(E) were not only able to transform the gas into a solid, but they were also able to create

The answer is C.