Here is the sentence again:

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 exam and poor causes a comma splice. At this spot, there are two complete sentences joined with a comma alone. William was oblivious ... begins the first complete sentence. Poor William was quietly earning a zero is the next complete sentence.

To fix this problem, you could put a period after exam and capitalize the P that begins poor.

Go to the next sentence.

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