One Saturday, Bob decides to take a drive through the countryside. After a few hours, he sees a dog walking behind a man on the side of the road. As the car comes near them, the dog jumps into the road. Of course, the car hits the poor animal and kills it. Bob stops his car and walks up to the man. "I'm very sorry," he says. "Is there anything I can do? Will $10 help?" "Oh, but...," says the man. "I should tell you..." "Stop! You're right. Maybe $10 isn't enough! That dog was your best friend!" Bob then takes $30 out of his pocket and gives it to the man. Then he thanks the man and drives away. After Bob leaves, the man looks down at the dog and says: "I wonder whose dog that is." From the passage, we know that   _   .
The dog is Bob's. The dog is the man's. The dog is the man's best friend. No one knows whose dog it is.