GMAT Sentence Correction (SC) | Discussion of greenhouse effects have usually had as a focus the possi

I don't understand whywhether Earth would grow warmer is parallel toto what extent :

>whether Earth would grow warmer is a dependant clause with a conjunction, a subject and a verb
>to what extent is is a prepositional phrase with a preposition and a noun

Moreover, the rootDiscussion.... has focused on...
a)...whether Earth would grow warmer works
b)...to what extent doesn't work

Thus, I'd conclude that the the Earth part and the extent part aren't parallel.

Any explanations?