题目内容：
根据下面短文内容，回答题。 Operation Migration
If you look up at the sky in the early fall in the northern part of North America, you may see groups of birds. These birds are flying south to places where they can find food and warmth for the winter. They are migrating (迁徙). The young birds usually learn to migrate from their parents. They follow their parents south, in one unusual case, however, the young birds are following something very different.
These birds are young whooping cranes, and they are following an airplane!
The whooping crane is the largest bird that is native to North America. These birds almost disappeared in the 1800s. By 1941, there were only about 20 cranes alive, In the 1970s, people were worried that these creatures were in danger of disappearing completely. As a result, the United States identified whooping cranes as an endangered species that they needed to protect.
Some researchers tried to help. They began to breed whooping cranes in special parks to increase the number of birds. This plan was successful. There were a lot of new baby birds. As the birds became older, the researchers wanted to return them to nature. However, there was a problem:
These young birds did not know how to migrate. They needed human help.
In 2001, some people had a creative idea. They formed an organization called Operation Migration. This group decided to use very light airplanes, instead of birds, to lead the young whooping cranes on their first trip south. They painted each airplane to look like a whooping crane.
Even the pilots wore special clothing to make them look like cranes. The cranes began to trust the airplanes, and the plan worked.
Today, planes still lead birds across approximately 1,200 miles (1,931 kilometers), from the United States-Canadian border to the Gulf of Mexico. They leave the birds at different sites. If a trip is successful, the birds can travel on their own in the future. Then, when these birds become parents, they will teach their young to migrate. The people of Operation Migration think this is the only way to maintain the whooping crane population.
Operation Migration works with several other organizations and government institutes.
Together, they assist hundreds of cranes each year. However, some experts predict that soon, this won't be necessary. Thanks to Operation Migration and its partners, the crane population will continue to migrate. Hopefully, they won't need human help any more.
Whooping cranes migrate in winter to__________. A．find warmth and food
B．raise baby whooping cranes
C．get human help
D．lay eggs
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