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

_______ he had not hurt his leg, John would have won the race.

--- How did you like the concert last night? --- I enjoyed it very much, but the dancers ______ a better job.

—— Could I use your telephone? —— Yes, of course you ＿＿＿＿＿＿＿＿＿＿＿＿＿. 

The teacher demanded all the exercise books ______ the next day.