题目内容：

    In habitats across the planet, animals periodically drop everything to walk, fly or swim to a new place. Wildlife such as whales and geese learn migration (迁移) paths by following their parents. Others, including small songbirds, gain the distance and direction of their migration within their genetic (遗传的) code. And some animals use a combination of genetics and culture to guide their migration.

Another group of migrators does not quite fit either model, and researchers have only recently started to figure out how they find their way. Take the Cory's shearwater, an oceangoing sea bird that migrates over the Atlantic every year. The young do not migrate with their parents, so culture cannot explain their journeys. And the exact paths vary wildly from individual to individual, making genetics equally unlikely.

Cory's shearwaters are long-lived, rarely producing young successfully before age nine. This leaves an opening for learning and practice to develop their migration patterns. Researchers call this the "exploration-refinement", and until now it has been hypothetical (假设的) because of difficulties in tracking migratory animals' movements.

But a team of researchers has done that by attaching small geolocators to more than 150 of the birds aged four to nine. They found that younger birds traveled longer distances, for longer periods, and had more diverse paths than older birds. “We finally have evidence of the "exploration-refinement" for migratory birds," says Letizia Campioni, who led the study. Younger Cory's shearwaters are able to fly just as fast as the adults- but they do not, suggesting that the young do more exploring, which gradually fades as they mature and settle into a preferred course.

“Although it may seem less efficient than other strategies, ‘exploration refinement’ could be beneficial to birds and other organisms in a rapidly changing world due to unpredictable man-made changes,” says Barbara Frei. “It might be safer to repeat a behavior that was recently successful than to rely on patterns that were perfected long ago but might no longer be safe.”

1.The first paragraph mainly        .

A.describes animals' habitats B.talks about migration models

C.compares different species D.introduces a tracking technology

2.What does the underlined word "this" in paragraph 3 refer to?

A.The long life of Cory's shearwaters.

B.The way Cory's shearwaters form their migration patterns.

C.The opening for learning and practice.

D.The process scientists track Cory's shearwaters' movements.

3.What docs Letizia Campioni's study find about the younger Cory's shearwaters?

A.They travel as much as adult birds.

B.They move in a predictable manner.

C.They lower the speed for exploration.

D.They look for a course with their parents.

4.What can we conclude from the last paragraph?

A.Man-made changes make migration easier.

B.Animals make a safer journey via a fixed track.

C.Course exploration contributes to birds' adaptability.

D.A combination of strategies assures migration success.