单项选择题
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
单项选择题 Why did one scientist expose birds to artificial daylight（）
单项选择题 The author states that birds left the tropics because （）
单项选择题 According to one theory, when the glaciers disappeared, birds（）
