根据句子意思，用括号中所给单词的适当形式填空。
1. The tennis player won a gold medal for his coming (twenty) birthday.
2. "I'll do everything I can to help you." She said with a (cheer) voice.
3. (lucky) for me the train was late, so I just catch it.
4. The (explore) of the solar system began in the 19th century.
5. By (use) the Internet you can do your shopping from home.
6. You had better (take) an umbrella-it's going to rain.
