You can ride your bike to school, but remember to be careful ____ .　　

This magazine isn't popular with the students though it____  every week.

It's difficult____  both the old and the young about the program at the same time.

The kids clapped and shouted ____ when the actor appeared.

Though he is very busy, he goes to the movies to relax____ 

The children are interested in playing the piano, so they need more____ 

The visitors ____  not to swim in this river for several times.

There are no students in the classroom. They ____  on the playground.

Thank you for____  the museum. We've learned a lot.

You must do more exercise, ____  twice a week.

如图为大气垂直分层示意图，据图回答下题．