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

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.