Which sentence in the passage contains a punctuation error?
A. (2) Sheldon, Leonard's roommate, was reading a thick textbook, and he looked up at his friend with a confused expression!
B. (1) "Sheldon, why is there a 'Danger' sign posted in front of the elevator?" asked Leonard as he entered the apartment.
C. (8) "Oh well, I'm sure it's nothing," Leonard told himself.
D. "I have been in my room all day reading my physics textbook," replied Sheldon.