How can Platypuses be mammals when they lay eggs?

Question

Which of these does not hold true for mammals?
[A] All Have a Backbone
[B] All have hair at some stage
[C] All Females have mammary gland
[D] All females give birth to live young

Answer 1

Mammals that lay eggs are called monotremes and include platypuses and echidnas, both of which live in Australia. Like all mammals, monotremes are warm-blooded, covered with fur and nurse their young. Animals are so interesting because they don't always follow the rules.