He saw a notice at the entrance ____ , “No spitting”[ ]

Only ______ the painting _______ decide whether it is worth buying.

Hi, Jack, would you like to join us in the sightseeing this weekend—Thank you, but I regret ______ you that I can’t, for I’m going to visit my grandparents.

下列各句中，加横线的成语使用恰当的一项是（ ）

If you are traveling _____ the customs are really foreign to your own, please do as the Romans do ?[ ]