根据句意和汉语注释，写出单词的正确形式。
The _____________ （建议）they have offered us are very useful, we should take them.
Beijing is the capital of China, it is also the _______________（文化）center of China.
Tonight, they are going to celebrate their father’s _____________ (四十) birthday.
How beautiful those circles on those ________________________ (蝴蝶)wings are!