单项选择题

Human sympathy (62) whales is only natural of all the creatures in the sea. (63) are closer relatives to us than these warm-blooded mammals. And how they got into the sea is one of the most fascinating stories of (64) Most authorities believe that 60 million years ago ancestors of modern whales were four-legged, wolf-size animals living on the sea shores, (65) an abundance of fish and shrimp tempted them to try wading. (63) 10 to 15 million years, their bodies grew, forelegs shrank into flippers used for balance and steering and hind legs disappeared. As a result of some amazing transformations, they are now helpless on land. If stranded on a beach, they can barely breathe.
With abundant (67) of food, whales grew into the largest creatures that lived, (68) larger than dinosaurs. A blue whale can grow to 100 feet. Its tongue is ten feet thick and heavier than an elephant. Some arteries are big enough for a child to swim (69) . The half-ton heart has walls two feet thick and pumps eight tons of blood. (70) its size comes awesome strength. A blue whale swimming (71) 15 knots generates 1 000 horsepower. (72) their size, these giants move at a good speed. An 18-ton whale can even (73) 12 m. p. h., over short distances. A whale can (74) to 9 000 pounds of food a day. The world’s biggest creature (75) itself almost entirely on shrimp--like krill smaller than a person’s thumb.
Maternal instincts are also (76) . Because a calf is born underwater, the mother must get it to the surface before it (77) Often another whale will help. The mother pushes it gently (78) the baby is confident with its swimming--usually after about 30 minutes. If the calf is (79) , she may support it on her back until it gradually rots away. Like all mammals, whale babies (80) mother’s milk. And the milk is more than 30-percent fat, over 10-percent protein, and the babies grow extremely fast. A blue whale calf lengthens (81) two inches a day and gains an average seven pounds per hour.