Take another look at the sentence:

Since he was sound asleep on the library sofa, William was oblivious to his twenty-eight algebra classmates, who were sweating, sighing, and wracking their brains as they tried to ace their final exam, poor William was quietly earning a zero.

The comma between sofa and William is completely correct. Since he was sound asleep on the library sofa is a subordinate clause, so it cannot stand alone. William was oblivious ... begins a main clause. When a subordinate clause introduces a main clause, you need a comma between them.

Go back to the sentence to try again.

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