Here's why:
* Rodents are mammals with two large, continuously growing incisors in the front of their mouths. Beavers fit this description perfectly.
* Marsupials are mammals that give birth to relatively undeveloped young who then complete their development in a pouch. Beavers are not marsupials.
* Egg layers are animals that reproduce by laying eggs. Mammals, including beavers, give birth to live young.