单项选择题
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
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.
A．must always be in credit B．can't draw any money if you're overdrawn C．can draw money without notice D．can't pay money to anyone else