If law and order as well as rules and regulations______, neither the citizen nor his property is safe.

You’re late! You were ____to be here an hour ago.— Sorry, Mr. Hu.

This nationwide smog should serve as a reminder to all, indicating a high time that we ______ on what we’ve done to the environment.

I’m not good at playing ________ piano, but I can play ________ volleyball very well.

You should not go rafting ________ you know how to swim