A．They have built a series of hummingbird feeding stations.
B．They have supported new laws that punish polluters of wildlife habitats.
C．They have replanted native flowers in once polluted areas.
D．They have learned to identify various hummingbird species.
A．Receiving radio signals from electronic tracking devices.
B．Being contacted by people who recapture banded birds.
C．Counting the birds that return to the same region every year.
D．Comparing old and young birds' migration routes.
A．The rocks cannot move after ice storms.
B．The rocks do not move at night.
C．The rocks never move in circles.
D．The rocks are not moved by people.
A．Their results were decisive.
B．They were not carried out carefully.
C．They were not continued long enough to reach a conclusion.
D．The government should not have allowed the experiments.