单项选择题
A．did B．does C．can D．will
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．did not feel guilty about wasting time B．were able to act of our own free will C．seemed to have better weather D．did not have so many enemies