问题详情：
根据句子意思，用括号内所给词的适当形式填空。
1． ‘I’ll do everything I can to help you.’ She said with a (cheer) voice.
2． On the _________ (40) Earth Day, Dr. Wang gave us a talk on the importance of protecting the earth.
3． The foreigner’s Chinese was very poor when he came to China, so he couldn’t make _________ (he) understood.
4． What an _________ (please) trip it was! Many visitors felt sick all the way.
5． The (explore) of the solar system began in the 19th century.
【回答】
1. cheerful 2. fortieth 3. himself 4. unpleasant oration
知识点：构词法
题型：完成句子
