GMAT Sentence Correction (SC) | Re: Many experts regarded the increase in credit card borrowing

Hello GMATNinja and other experts

In the explanation given by official guide for this question, it says that affirmed clause is incorrectly introduced by rather, yet, and but.
What is an affirmed clause? Apart from correct word pair "not as......but as...." , can any other word pair or just a word be used here?

Thanks

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