To which class does the bat belong?

Which of the following animals possesses a four-chambered heart?

$A$: The duck-billed platypus and the spiny ant-eater,both are egg-laying animals yet are grouped under mammals.
$R$: Both of them have seven cervical vertebrae and mammary glands without teats.

In which class of mammals is a marsupium or brood pouch found?

Point out the mammalian characters.

Which aortic arch is present in mammals?