Answer:
They give birth to young, have fur, have lungs, and provide milk for the young.
Step-by-step explanation:
According to the UCSB Science Line, Whales are mammals because they give birth to young, have fur, ( even though it may not be visible ) have lungs and breathe, and provide milk for the young.
I got these facts from the UCSB Science Line, you can check it out if you want.
Hope this helps! <3