阅读短文，下列问题
People sometimes like to read stories of dogs very much. They think that dogs are much cleverer than cats, sheep, cows or other animals in their homes.
One of my close friends, Bob, has a very large police dog named Jack. Every Sunday afternoon, Bob and Jack have a walk in the park nearby. Jack likes these walks very much. One Sunday afternoon, I paid a visit to my friend. I stayed there for a long time and my friend and I had much more talk with each other than ever before. Soon it was time for them to take a walk in the park. We forgot that. Jack became worried about it. He walked around the room several times and then sat down in front of me and looked at me. But I still paid no attention (注意) to him. I went on talking with my friend. At last, Jack could not wait any longer. He went out of the room and came back a few minutes later. He sat down in front of me again. But this time, he held my hat in his mouth. Suddenly, I understood what Jack meant and so did my friend.
臭氧。下列说法正确的是（ ）