编写一个函数，输入为一个字符串，输出为将该字符串中的所有单词的首字母大写的新字符串。


"hello world, how are you?"
```python
def capitalize_words(string):
   words = string.split()
   capitalized_words = [word.capitalize() for word in words]
   return " ".join(capitalized_words)
```

例如：`capitalize_words("hello world, how are you?")` 的输出为 `"Hello World, How Are You?"`