Some physics problems in this book are _________difficult for the students _________work out.

Frank often wears a mask _____ he can keep fit in the foggy and hazy (霾) weather.

I’d like to go with you, _____________I’m too busy.

The bus driver always says to us: “Don’t get off ______ the bus stops.”

They are in the same school，_________in different classes．
A．and          B．or           C．but

Study hard, _        _ you will pass the exam.

Diana isn’t here, ______ leave a message on her desk.
A. or                   B. so               D. and                  D. but

He’s not a perfect child. He sometimes talks back ____ his parents talk with him.

John didn’t give up looking for a job __________ he got an offer from a German company.

— Would you like to go to the cartoon show with me?
— It sounds like fun, ______I’m too busy.