Take another look at the item:

Gloria and Scooby stood in front of the bathroom mirror, (A) bitterly complaining about the difficult midterm and stern teaching in their biology course. (B) As Mrs. Perry, their professor, (C) eavesdropped from inside a far stall.

You wanted to do this:

Gloria and Scooby stood in front of the bathroom mirror bitterly complaining about the difficult midterm and stern teaching in their biology course. As Mrs. Perry, their professor, eavesdropped from inside a far stall.

Removing the comma after mirror creates an error. Bitterly complaining about the difficult midterm ... is a participle phrase. You need a comma to connect it to the main clause in front.

You might want to review the rules.

Go back to the sentence to try again.

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