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1·Do not drop litter carelessly.

不要粗心大意地扔垃圾。

2·Don't drop litter carelessly.

不要随便丢垃圾。

3·You can't drop litter carelessly.

你不能乱扔垃圾。

4·You 'd better not drop litter carelessly.

你最好别随意扔垃圾。

5·People ought not to drop litter carelessly.

我们不应该随便扔垃圾。