1. 根据句意和汉语注释，写出单词的正确形式。

1. （1） I didn't （意识到）what happened until he told me about it.

3. （2） Many Chinese people who worked （在国外）have come back to China to find jobs.

5. （3） Snow has prevented （沟通）with the outside world for three days.

7. （4） Water （污染）is one of the most serious problems we are facing now.