No single adjective is adequate to describe the size of the blue whale, and few people realize how it compares with other mammals. By any standard, it is the largest creature known to man. To be specific, one of its fins, called a fluke, would fill the cargo space of the average dump truck. Although its skull is the size and weight of a car, its brain is only the size of a carburetor. Its heart is so large that five strong men would be needed to lift it, while its skin could be used as a tarpaulin to cover half a football field. Every person in Boston could be supplied with a whaleburger, and a pretty good one, too, from the meat of one blue whale. Combine the weights of 100 horses, 100 dairy cows, and 5 Indian elephants, and you would arrive at the weight of one female blue whale. Fortunately for the whale, its watery environment has protected it from the extinction suffered by the dinosaur millions of years ago.

Q. Underline the topic sentence of the paragraph.