Is the whale the world’s largest fish? No. It looks like a fish, but it is really a mammal (哺乳动物). Then what’s the world’s largest fish? It’s the whale shark.

Although the whale shark is our biggest fish and lives in many oceans around the world, few people have ever heard of it. Scientists have not been studying whale sharks for very long. Much more needs to be learned about them.

A whale shark is sometimes called a “gentle giant (温柔的巨人)”. It likes to be alone and doesn’t care a bit about divers or humans. The biggest whale shark weighed 24,250 pounds and was nearly 40 feet long. This makes it heavier and longer than a school bus.

Inside its huge mouth are great numbers of very small teeth. To count all of them, you’ll need to do some math. The whale shark’s teeth are arranged in rows. There are between eleven and thirteen rows on each jaw (颚) and around three hundred teeth in each row.

However, the whale shark rarely uses its teeth. It does not chew or bite its food. Large amounts of water are pulled into its mouth and then pushed out through its gills (鳃).

For a few months every year, a group of whale sharks comes to Ningaloo Reef, off Western Australia. This gives scientists a wonderful chance to study the large fish. Whale sharks do live in other oceans of the world, but no one knows their population. It is possible that our biggest fish may need protecting. It is one of the reasons why scientists continue to learn all they can about these gentle giants.

51. Little is known about whale sharks because scientists _____.

A. are not interested in learning about them

B. find them too difficult to catch

C. do not know the best places to find them

D. have studied them for a short time

52. Which sentence can tell us that whale sharks are gentle?

A. They have large mouths.

B. They leave divers alone.

C. They have small eyes.

D. They live in many oceans.

53. How does the writer help us know what the whale shark’s mouth looks like?

A. By giving reasons.

B. By describing colors.

C. By comparing sizes.

D. By using numbers.

54. The fifth paragraph is mainly about _____.

A. how a whale shark eats

B. what a whale shark eats

C. how different fish use their gills

D. what’s inside a whale shark’s mouth

55. Which of the following is the best title for the article?

A. Gentle Giants                                         B. The Diary of a Sailor

C. The Largest Animal                                 D. My Life on the Sea