单项选择题

Sharks
Sharks are amazing fish that have been around since long before the dinosaurs existed. They live in waters all over the world, in every ocean, and even in some rivers and lakes.
Sharks and Bony Fish
Unlike bony fish, sharks have no bones; their skeleton is made of cartilage(软骨), which is a tough, fibrous substance, not nearly as hard as bone. Cartilage, a strong fibrous(纤维的) substance, is softer than bone; our nose and ears are made of cartilage. Sharks also have no swim bladder (unlike bony fish).
Size
There are many different species of sharks that range in size from the size of a person’s hand to bigger than a bus. The whale shark is the largest fish in the world; the basking shark is the second largest fish. Fully-grown sharks range in size from 7 inches long (the Spined Pygmy shark), up to 50 feet long. Most sharks are intermediate in size, and are about the same size as people, 5 - 7 feet long. Half of the shark species are under 39 inches long.
Variety of Sharks
There are about 368 different species of sharks, which are divided into 30 families. These different families of sharks are very different in the way of look, live, and eat. They have different shapes, sizes, colors, fins, teeth, habitat, diet, personality, methods of reproduction, and other attributes. Some types of shark are very rare and some are quite common. The spiny dogfish shark is the most common shark.
Body Shapes
Sharks have a variety of body shapes. Most sharks have streamlined (流线型的), Some sharks have an elongated body shape (e. g. cookiecutter sharks and wobbegongs). Sawsharks have elongated snouts, thresher sharks have a tremendously elongated upper tail fin which they use to stun prey, and hammerheads have extraordinarily wide heads. The goblin shark has a large, pointed protuberance (突出) on its head; its purpose is unknown.
Teeth
The teeth of sharks are also striking. Sharks may have up to 3,000 teeth at one time. Most sharks do not chew their food, but swallow it down whole or in large pieces. The teeth are arranged in rows; when one tooth is damaged or lost, it is replaced by another. Most sharks have about 5 rows of teeth at any time. The front set is the largest and does most of the work.
Diet
Sharks vary greatly in their diets, but they are all carnivores(食肉动物). Some eat fish, other sharks, and marine mammals; some eat shellfish from the ocean floor; and others eat tiny bits of plankton(浮游动物) and small animals from the water as they swim with open mouths. They eat huge amounts of these tiny animals and plants.
Sharks Attacks
When some sharks (like the Great White or the Gray Reef shark) turn aggressive prior to an attack, they arch their back and throw back their head. They also move their tail more acutely (probably in preparation for a chase).
Sharks do not normally attack people, and only about 25 species of sharks are known to attack people. Sharks attack fewer than 100 people each year. Many more people are killed by bees or lightning.
The sharks that are the most dangerous to people are the great white shark, the tiger shark, the bull shark, and the oceanic white tip shark. The bull shark is the most frequent attacker of people as it swims in very shallow waters where people swim and is a very plentiful shark. Some of the other sharks that are known to have attacked people include the gray shark, blue shark, hammerhead shark, mako shark, nurse shark, lemon shark, black tip reef shark, wobbegongs, sandtiger, spitting sharks, and the porbeagle. Some people believe that sharks mistake people (especially people swimming on surf boards) for seals and sea lions, some of their favorite foods.
Occasionally, a group of sharks will attack a food source (for example, a school of fish) in a maniacal fashion. They will wildly attack the food and anything in the area, even each other, sometimes wounding or eating fellow sharks.
Habitat
Sharks live in oceans and seas all over the world, and even in some rivers and lakes, especially in warmer waters. Some sharks live near the surface, some live deep in the water, and others live on or near the ocean floor. Pelagic(远洋的) sharks (living in the open ocean) include the great white shark, the basking shark, etc. Benthic sharks (living at the ocean floor) include the zebra horn shark, the wobbegongs, and the angelshark, which usually have flattened, camouflaged(伪装的) bodies that let them hide in the sea bed. Some sharks even venture many miles up into the fresh water of rivers like the Mississippi in the USA and the Amazon in Brazil. The bull shark (Carcharhinus leucas) sometimes ventures into fresh water.
Migration of Sharks
Some sharks live in relatively warm waters (hammerheads, bull sharks, and tiger sharks). Other sharks, such as the thresher, mako, basking and blue shark, live in temperate water (which is neither hot nor cold). Others, including the dogfish, Greenland, and goblin, live in cool waters. Some sharks stay in the same region in their entire lives while others travel across oceans. There are three different types of sharks when it comes to migratory(迁移的) patterns: Local sharks, coastal pelagic sharks, and highly pelagic sharks. Local sharks do not migrate, and range only about a hundred miles from their habitat, coastal pelagic sharks can migrate over 1,000 miles, while highly pelagic sharks migrate across oceans.
Evolution of Sharks
Sharks have existed for over 350 million years. They evolved over 100 million years before the dinosaurs did. This was long before people evolved. Most fossil evidence of early sharks is from fossilized teeth and a few skin impressions. Cladodonts, primitive sharks, had double- pointed teeth, were up to 6 feet long and lived about 360 million years ago; they ate fish and crustaceans(甲壳类).
Megalodon was an ancient, meat-eating shark, living between 25 - 1.6 million years ago; it is extinct. It was over 40 feet long, but this is only an estimate from fossil teeth that have been found. Its teeth resemble those of the great white shark but are almost 3 times larger; these teeth are each the size of a person’s hand! Why may the bull sharks attack people frequently