23（）

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

A．to help children to gain a better understanding of animal life
B．to prove that the saying "a rolling stone gathers no moss" is wrong
C．to introduce a particular animal to the common reader
D．to discuss with professionals the laziness of animals

A．To test the relationship between daylight and a disease of the glands common to birds.
B．To test the relationship between daylight and migration.
C．To test the relationship between migration and temperature.
D．To test the relationship between daylight and changes in the season.