Take a look at the item again:
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 Revolution makes a fragment. While Professor Koopman explained ... is a subordinate clause and cannot stand alone as a complete sentence.
You might want to review the rules.
Go back to the sentence to try again.