C

Is it to their body temperature? Save energy? Find food? Tiger sharks at Ninglaoo Reef are thought to search the seafloor for prey(猎物) as they dive down and scan for direction as they swim up to the surface. But could there be other reasons why the sharks continuously move up and down?

That's what Sammy is hoping to find out for her Ph.D. research. Sammy attached tracking devices to 24 tiger sharks. Best described as Fitbits for sharks, the devices recorded activity rates and other data 20 times a second. Fitbits also contained video cameras, so Sammy could see the habitats the sharks moved through and the animals they met. She watched how the sharks reached to prey and how the prey reached to them.

Tiger sharks can be pretty lazy --- something as simple as a turtle noticing a shark and tuning away could cause the shark not to bother hunting it. Stomach content analysis has found their normal prey items, such as turtles, rays and fish. But it has also found some really interesting things, such as licenses, cans and nails.

Sammy says that studying tiger sharks can help her better understand the ecosystem as a whole. How they move through water and feed can help her figure out how they might be influencing animals beneath them in the food chain

"My time watching tiger sharks hunt was the best month of my Ph.D., if not any life." Sammy says. "At first, I was a bit apprehensive about it......getting that close to some very big animals. But they were very chilled(温驯) once you had them controlled alongside the boat. They'd just sit there, you'd attached the tracking devices, you 'd take the line off and they'd just swim off really calmly, it was pretty amazing to see. They're just absolutely beautiful animals."

28. Why does Sammy do her research?

A. To find some scientific data.

B. To develop others' interest in tiger sharks.

C. To find out how tiger sharks move.

D. To further understand tiger sharks' moving.

29. What does Sammy think of studying?

A. It's of great benefit.

B. It's a tiresome job.

C. It's filled with challenges.

D. It's simple and calls for little efforts.

30. What does the underlined word "apprehensive" in the last paragraph mean?

A. Curious. B. Doubtful.

C. Fearful D. Enthusiastic.

31. Which can be the best title for the text?

A. How do tiger sharks find food?

B. Why do tiger sharks dive?

C. What can we do to protect tiger sharks?

D. What roles do tiger sharks play in the food chain?