The Migration of Birds Here is a scientific experiment on the homing of birds, the fact of

The Migration of Birds
Here is a scientific experiment on the homing of birds, the fact of which are quite certain. A few years ago seven swallows were caught near their nests at Bremen in Germany. They were marked with a red dye on some of their white feathers, so that they could easily been seen. Then they were taken by plane to Croydon, near London. This is a distance of 400 miles.
Then the seven swallows were set free at Croydon. Five of them flew back to their nests at Bremen. How did the birds find their way on that long journey, which they had never made before? That is the great puzzle. It is no good saying that the swallows have a sense of direction. These are just words and explain nothing. We want to know exactly what senses the animals use to find their way, how they know in which direction to go until they can see familiar landmarks. Unfortunately practically no scientific experiments have yet been made on this question.
Perhaps migrating birds are the greatest mystery of all. Swallows leave England in August and September, and they fly to Africa, where they stay during our winter. The swallows return to England in the late summer for the south. A lot has been found about the journeys of migrating birds by marking the birds with aluminum rings put on one leg. An address and a number is put on the ring.
Swallows from England go as far as South Africa and as many as fourteen birds, marked with rings in England, have been caught again in South Africa. From England to South Africa is a journey of 6,000 miles. And the birds not only return from Africa to England next spring, but often they e back to the nests in the very same house where they nested the year before.
17．The seven swallows were marked on some of their feathers because       .
A．they would be taken away by plane
B．they would be free in London
C．Croydon was 400 miles away
D．they could easily be seen for the red colour
18．Which of the following is true?       .
A．Seientists have found why the animals can see their nests.
B．No one knows why the swallows can fly back to their familiar landmarks.
C．It’s very simple that the birds use sense of direction.
D．Many experiments have been made on how birds can fly their way home
19．Swallows like to spend ”our winter” in        .
A．England      B．London        C．Africa       D．Germany
20．When do the birds return from South Africa to England every year?
A．summer        B．spring        C．winter         D．Both A and B17---20     DBCD