Sharks are among the oldest animals on Earth. There are hundreds of kinds of sharks. Most are about two meters long. The dogfish shark, however, is less than twenty centimeters in length. A shark has an extremely good sense of smell. It can find small amounts of matters in water, such as blood, body liquids and chemicals produced by animals. These powerful senses help sharks find their food. Sharks eat fish, other sharks, and plants that live in the ocean. Sharks grow slowly. About forty percent of all sharks lay eggs. The others give birth to live young. Some sharks carry their young inside their bodies as humans do. Some sharks are not able to reproduce until they are twenty years old. Most reproduce only every two years. And they give birth to fewer than ten young sharks. For this reason, over-fishing of sharks is of special danger to the future of the animal. Sharks are important for the world's oceans. They eat injured and diseased fish. Their hunting activities mean that the numbers of other fish in ocean waters do not become too large. This protects the plants and other forms of life that exist in the oceans. People hunt sharks for sport, food, medicine and their skin. Experts say the international market for some kinds of sharks has increased because many parts of a shark are valuable.Collectors pay thousands of dollars for the jaws of a shark. Shark liver oil is a popular source of Vitamin A. The skin of a shark can be used like leather. In Asia, people enjoy a kind of soup made from shark fins. Experts say a fisherman can earn a lot of money for even one kilogram of shark fins. Sharks are among the oldest animals on Earth, but some sharks are in danger of disappearing from Earth. If too many sharks in one area are killed, that group of sharks may never return to normal population levels. According to the passage, many parts of a shark are valuable EXCEPT   _  .
its jaws its liver shark fins shark teeth