请根据句意从方框中选择合适的单词，并用其适当形式填空，使句子通顺。

pollute     twenty     rule   direct     communicate

【小题1】Don’t worry. The exchange students will ________ fly to Hong Kong. They still have enough time.
【小题2】We can’t drink the ________ water from the river.
【小题3】You can find all the ________ names of Qing Dynasty in this book.
【小题4】The problems between parents and their children sometimes come from bad ________ skills.
【小题5】China has known more about the world in the early ________ century.