根据所给的汉语提示，用合适的单词填空。
---Do you know Hong Kong? ---Yes，it ____________ (回归)to China in 1997.
Lucy and Tom have been in love for many years and they’ll get ____________(结婚的)next month.
They all think it’s____________ (不可能的)to finish the work in such a short time.
How to protect the ________________（环境）has been under discussion.
The fans _______(鼓掌)and screamed when the famous singer appeared on the stage.