---I am going to Nanjing and stay there for a week. ---_________you are there, would you please buy some books for me?
—We’ll make a trip to Hainan Island next weekend. Will you go with us? —No, I can’t _____ it at present because I have little money.
—How many students are there in the classroom? —_____. I don’t know where they have gone.
The rubbish _____ is produced every day is becoming a serious problem in cities around the world.
More and more trees were cut down. _____, many animals are dying out.
We should often keep _____ touch with each other, because we are good friends.