请根据句意从方框中选出合适的词，并用其适当形式填空，使句子通顺
…important    culture    active    thirty-three    necessary…
1. The principal gave us a talk on the            of protecting the environment yesterday.
2. All of us are grateful to him because he always            helps us solve problems.
3. It’s             to tell him the details of the trip. He won’t go with us tomorrow.
4. There are many history and              differences between China and England.
5. ―Who is the              President of the USA?  
―Harry S. Truman.