Chief distinguishing features of the mammals are

What is common among whales,bats,and rats?

Egg-laying mammals are found in:

All of the following are mammals that live entirely in water,except:

Assertion : The duck-billed Platypus and the spiny ant-eater,both are egg-laying animals yet they are grouped under mammals.
Reason : Both of them have seven cervical vertebrae and $12$ pairs of cranial nerves.

Which one of the following is an exclusive character of class Mammalia?