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Because that's what the people involved in producing it decided to do. There's not some kind of conspiracy and I doubt a great amount of deep thought went into it.
Yes. Mammals are the only animals that feeds milk to their young.
no
No! Fish, reptiles, birds, and amphibians all have backbones, but are not mammals.
Only monkeys are mammals among those animals you named.
No, domestic animals can include birds, reptiles or fish.
Milk teeth would, by definition only occur in mammals, since mammals are the only animals that produce milk for their young. Other animals may well have a set of deciduous teeth that are replaced by adult teeth as they mature.
No. Only mammals have belly buttons.
Yes. Only birds and mammals are warm-blooded.
humans are mammals, so if there were no animals, the only life on Earth would be plant life
Mammals are the only vertebrate group which feed their young on mothers' milk.
the mammals that is the only thing i know