Read the text and answer the question!

    Whales are sea-living mammals. They therefore breathe air but cannot survive or land. Some species are very large indeed and the blue whale, which can exceed 30 meter length, is the largest animal which lives on earth. 

    Superficially, the whale looks rather like a fish, but there are important difference in its external structure; its tail consists of a pair of broad, flat horizontal paddles (the tail of a fish is vertical) and it has a single nostril on top of its breadth, broad head. The skin is smooth and shiny and beneath it lies a layer of flat (blubber). This is up to 30 meter in thickness and serves to conserve heat and body fluids. 

Sentence 1 ...