用适当的数词完成下列句子?
1.One h（ ）years is a century.
2.Thursday is the f（ ）day of the week.
3.Letter H is the e（ ）letter in the alphabet.
4.There are t（ ）months in a year.
5.MY brother is just 20.Today is his （ ）【20岁】birthday.