Bubby has got a doorbell. If you visit him, you must ring his doorbell.
One day, Mr. Panda visited him. He knocked at his door. He knocked and knocked. After a while, Bubby shouted in the room, "Please ring the doorbell!" But Mr. Panda didn't hear it. He knocked again. This time, Bubby opened the door and said, "Don't knock at the door. I can't hear. Ring the doorbell, please."
【小题1】Bubby doesn't have a doorbell. (_____)