What is the criteria used to classify mammals into groups? based on the type of food they eat
based on their appendages used for movement
based on how their embryos develop
based on their type of hair

The criteria used to classify mammals into groups is the presence of hairs.

Mammals:

They are the animals who have mammary gland mean the mother feeds the milk to their newborn.

One unique characteristics of the mammal is that they have have hair in at least one stage of their development.

No other animals like reptiles or birds have true hairs.

To know more about Mammals,

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based on how their embryos develop

Explanation:

Mammals are further classified into three types: placental, marsupial, and monotreme.

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✿   this landform has a high elevation and a high relief

✿   this landform is a mountain