Which of the following class is being described by the given statements? (i) They are found in a variety of habitats- polar ice-caps, deserts, mountains, forests, grasslands and dark caves. (ii) Most unique characteristic is the presence of mammary glands by which the young ones are nourished. (iii) Heart is four-chambered. (iv) Sexes are separate and fertilisation is internal.

• A. Reptilia         
• B.        Aves
• C. Mammalia      
• D.        Amphibia