用括号内所给词的正确形式填空。

1. The Chinese government has made great contributions to the _______

   of the world's high speed railway. (develop)

2. It's _______ possible that robot teachers will be popular in schools some day. (true)

3. Simon is such a _______ person that no one believes him.(honest)

4. This is the _______ and thinnest of all the e-books in the world. (light)

5. Our planet, Earth, is becoming more and more crowded and _______. (pollute)