根据短文内容，从短文后的七个选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。
Mass Migration   


Look up at the sky one autumn day and you might see a flock of birds in V-formation.    All summer, they have lived in the north where they made their nests, laid their eggs, hatched, and raised their young. Soon, snow and ice will cover all their northern breeding grounds, so they must migrate south where food is easier to find. It is necessary to create worldwide bird sanctuaries(保护区) before many of these species become extinct（灭绝）.
     The Arctic tern travels 10,625 miles from one polar region to the other. This may be the longest migration of any animal on Earth. Not all birds migrate. Some are able to adapt to winter by changing their diets.   For most species, however, it is easier to migrate to climates that are not as severe.
Somehow, migrating birds know exactly where they are going and how to get there. It is amazing to think about how birds are able to return to the same place year after year.     Other scientists believe that migratory birds navigate by following certain coastlines, mountains, and rivers. However they do it, these amazing creatures always arrive in the right place at the right time. In the spring, they return to their nesting grounds up north to lay their eggs and care for their young.
It is important for the people of the world to protect the estuaries (河口)and wetlands .      With so many species of birds’ habitats threatened worldwide, our children may never be able to enjoy the beauty of these migrating wonders.

A．These birds stay in the same place all year.

B．They are important to endangered species.

C．Some birds travel great distances, crossing entire continents and oceans.

D．They are flying south where there is warmth and food during the winter.

E. Birds fly around the world according to the weather year around year.
F. Some scientists think it is possible for birds to use the stars to find their way.
G. The birds that migrate south in winter return to different places when weather is warm.