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!