题目内容：
共用题干第三篇
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 1 800s.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 tired 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 approxlmately 1,200 mniles(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 whlooping
crane population.
Operation Migration works with several other organizations and government institutes.Together,they as-
sist 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 are native to_________.
A:Mexico
B:South America
C:the Persian Gulf
D:North America
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