theory   

It is reported that in our country cities ________ heavy air pollution that is 30 percent above the national standard or higher should try to meet standards by 2030.

He says whatever he thinks, ______ other people’s feelings.

It ______ me all of a sudden that I had left my key to the car in the office.

“After World War II, many of the cities across western European countries ________,” our guide said.

---Why do you look so upset?
---There are so many troublesome problems _____.

The accident has _____________to be a good thing after all．