阅读理解。

     The old man is already sixty years old. But he has only fifteen birthdays. How is it? When is his

birthday?

    Keys: His birthday is on February 29th. It’s a leap(闰年) year every four years. So he has only

fifteen birthdays.

根据短文内容填空:

1. The old man is _______ years old.  

2. He has only ________ birthdays.

3. His birthday is on _______.

4. A leap year happens(发生) every _______ years.