Does anyone know why the "o" is needed?
because it is "one hundred"
It should be "the hundredth dog. " this English sentence does not make sense
It is the hundredth dog,
It is the one hundredth dog. Same meaning every time.
Get with the program Duolingo!!
Learn proper English.
"It is the one hundreth dog" should be accepted.
I've heard of the hundredth monkey, but the hundredth dog?