—Could you please have a walk with me?—Sorry, I _______. I have something important to do now.

 —Boy, you _______ lean (伸) out of the window while the bus is running. That’s dangerous.—Sorry, I won’t do that again.

I’m impressed with his determination. When he had a problem to solve, he  _____ work at it until he found an answer.

The teacher told us the best way _____ English. 

—Why can your neighbor know so much about London?—He ____ have been there before.