根据短文内容填空

     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.

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.

某一四跨连续主梁，当欲求第二跨跨中最大正弯矩时，活载应布置于第二，四跨；当欲求第一内支座最大负弯矩时，活载应布置于（）跨。