Which sentence best indicates that "Object Lesson, Part 1" is a mystery

A. They're only patterning their behavior after criminals because they think criminals are strong, successful, and glamorous.
B. Class, This is Ellery Queen. I don't have to tell you who Mr. Queen is, and how honored we are to have him visit us.
C. An envelope with seven one-dollar bills in it was stolen, and from the way it happened, the thief can only be someone in this room.
D. [W]hen I saw the money wasn't in the envelope, my first thought was exactly that, Mr. Queen. So I searched myself thoroughly.