People should think about _____ they can have a positive influence on the planet.
John was so sleepy that he could hardly keep his eyes ___.
--___ do you want? --I want a book on history.
Shortly after the accident, two ___ police were sent to the spot to keep order.
Mr. Wang said that he ___ next year.
In the dark street, there wasn’t a single person ___ she could turn for help.