阅读理解阅读下列短文，从每题所给的四个选项(A、B、C和D)中，选出最佳选项。Althoughnooneiscertainwhymigrationoccursthereareseveraltheories.Onetheoryisbaseduponthestatementthat

Question

阅读理解

阅读下列短文，从每题所给的四个选项(A、B、C和D)中，选出最佳选项。

　　Although no one is certain why migration occurs there are several theories. One theory is based upon the statement that prehistoric birds of the Northern Hemisphere (半球) were forced south during the Ice Age when ice covered large parts of Europe Asia and North America. As the ice of rivers melted the birds came back to their homelands spent the summer and then went south again as the ice advanced in winter. In time the migration became a habit and now although the glaciers have disappeared the habit continues.

　　Another theory proposes that the ancestral home of all modern birds was the tropics. When the region became overpopulated many different sorts of birds were crowded north. During the summer there was plenty of food but during the winter short supply forced them to return to the tropics.

　　A more recent theory known as photoperiodism suggests a relationship between increasing daylight and the feeling of certain glands (腺) in the birds' bodies that may prepare them for migration. One scientist has been able to cause midwinter migrations by making birds uncovered to unreal periods of daylight. He has concluded that changes occur in the bodies of birds due to seasonal changes in the length of daylight.

1．According to one theory when the glaciers disappeared birds ________.

[　　]

A．stopped migrating

B．began migrating again

C．continued migrating

D．migrated south and stayed there

2．The author states that birds left the tropics because ________.

[　　]

A．there was not enough food there in the winter

B．there were too many birds

C．there were too many glaciers

D．there was too much daylight

3．Why did one scientist expose birds to artificial daylight?

[　　]

A．To test the relationship between daylight and migration.

B．To test the relationship between daylight and a disease of the glands common to birds.

C．To test the relationship between migration and temperature.

D．To test the relationship between daylight and changes in the season.

4．According to the theory of photoperiodism ________.

[　　]

A．birds should migrate in the middle of the winter

B．longer days cause changes in the bodies of birds

C．seasonal changes in the length of days do not affect migration

D．increasing daylight increases the distance of migration

5．This passage supports the belief that ________.

[　　]

A．exact reasons for migration are not known

B．birds migrate because of changes in temperature

C．the ancestral home of all birds was the tropics

D．glaciers caused birds to migrate

Answer 1

答案：C;B;A;B;A