Question

All mammals without any exception, are characterised by
A. Viviparity and biconcave red blood cells.
B. Extra- abdominal testes and a four- chambered heart
C. Heterodant teeth and 12 pairs of cranial nerves.
D. A muscular diaphragm and milk producing glands.

Answer 1

Hint: Most of the mammals are viviparous but some of them are oviparous. Some of the mammals do not have fully developed four chambered hearts. Higher vertebrates have 12 pairs of cranial nerves. All the mammals have mammary glands and muscular diaphragms.

Complete answer:
Mammals are the group of animals constituting the class Mammalia. Mammary glands are present in females to produce milk for feeding their young ones. Mammals give birth to their young ones except some of them. Mammals are classified as marsupials (give birth to early stage fetuses) and placental mammals (give birth to fully developed fetuses). Most of the mammals are viviparous as they give birth to young ones but some of the mammals such as Duck-Billed Platypus, short beaked Echidna etc are egg laying mammals. Camels have oval nucleated blood cells.
Spermatogenesis takes place in seminiferous tubules of testes. Scrotum is present in males. Birds, mammals and crocodiles have four chambered hearts that means their heart is separated into two atria and two ventricles.
Most of the mammals are heterodont that means they have a variety of tooth shapes: incisors, canines, premolars, and molars. Higher vertebrates have 12 pairs of cranial nerves.
Mammals have dome shaped muscles and have muscular diaphragm, mammals have mammary glands to feed their young ones.
From the above information we can conclude that all the mammals have muscular diaphragm and milk producing glands without any exception.

So, option D is the correct answer.

Note: Mammals are warm blooded animals as their body temperature remains the same all the time. Mammals nourish their babies and also provide protection and training to them. They have hair and fur on their body. Brain is well developed in mammals. Mammals are also known as tetrapods because they have limbs to move around. About 6,400 species of mammals have been described till now.