. As a teacher, he has never given his students so difficult a problem ____ they can’t work out.
The old city wall ______ in the Ming Dynasty.
The doctors have done ______ they could ______the child.
The teacher asked the students ______ .
I don’t know where I ______ my dictionary, which cost me 50 yuan.
If you are traveling___ the customs are really foreign to your own, please do as the Romans do.