I saw some boy students       _ basketball when I passed the playground.

I’d rather ________ a teacher when I grow up．

---I am sorry to have kept you ___________.---It doesn't matter. I've just come.

There is a paper factory _____ waste water into the river nearby.

 —We must stop ________ plastic bags (塑料袋).—You’re right. It’s bad for our environment(环境).