Which of the following are the unique features of mammals?

a) Mammary glands

b) Hair on skin

c) Pinnae

d) Heterodont teeth

e) Viviparity

a) Mammary glands - Mammals are characterised by the presence of mammary glands, which produce milk to nourish their young.

b) Hair on skin - Mammals have hair or fur on their skin, a distinctive feature that sets them apart.

c) Pinnae - Pinnae refers to the external, visible part of the ear in mammals. Not all mammals have large, visible ears, but the presence of pinnae is a unique feature.

d) Heterodont teeth - Mammals typically have different types of teeth (incisors, canines, molars) that serve various functions in processing different types of food. This is known as heterodont dentition.

e) Viviparity - Viviparity refers to the reproductive characteristic of giving birth to live young rather than laying eggs. While viviparity is a common feature among mammals, it is not exclusive to them.While some mammals ((platypus and echidna) are oviparous (lay eggs), the majority are viviparous. Some reptiles, such as certain species of snakes and lizards, also exhibit viviparity.