GMAT Sentence Correction (SC) | Re: There is generally agreement that feral swine are unwelcome

You don't necessarily need to have a full independent clause (subject + verb) after a comma + "but". I can think of at least a couple of situations in which the comma + "but" isn't followed by a full independent clause. Both situations involve parallelism:

• There are many species of rodent that are incredibly cute, but also incredibly vicious when threatened.
• Many invasive species of fish are not only damaging to local ecosystems, but also unappealing to commercial and recreational fishermen.

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