Take another look at the item:

While Professor Koopman explained the consequences of the French Revolution, (A) Tiffany and Faith sniffled uncomfortably in their seats, (B) suffocated by the cheap shaving lotion (C) that Henry always wore.

You wanted to do this:

While Professor Koopman explained the consequences of the French Revolution, Tiffany and Faith sniffled uncomfortably in their seats. Suffocated by the cheap shaving lotion that Henry always wore.

A period after seats makes a fragment. Suffocated by the cheap shaving lotion is a participle phrase and cannot stand alone as a complete sentence.

You might want to review the rules.

Go back to the sentence to try again.

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