单项选择题
A．it senses a coming danger B．it is forced to swim on a hot day C．it is short of its favorite food D．it wants to keep itself to itself
A．And B．Still C．Even D．So
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.