The category "<hour>" can be expressed as follows:
zero
oh
one
two
three
four
five
six
seven
eight
nine
ten
eleven
twelve
thirteen
fourteen
fifteen
sixteen
seventeen
eighteen
nineteen
twenty
zero zero
oh oh
oh one
zero one
oh two
zero two
oh three
zero three
oh four
zero four
oh five
zero five
oh six
zero six
oh seven
zero seven
oh eight
zero eight
oh nine
zero nine
one zero
one oh
one one
one two
one three
one four
one five
one six
one seven
one eight
one nine
two zero
twenty one
two one
twenty two
two two
twenty three
two three