GMAT Sentence Correction (SC) | The hognose snake puts on an impressive bluff, hissing and

Your understanding is alright. The sentence consists of 2 independent clauses:

1. The hognose snake puts on an impressive bluff.
BUT
2. it has no dangerous fangs and no venom and will fall over and play dead.

Note the nesting of verbs in the second clause: "Has" and "Will fall and
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