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.
A．It has been the site of Earth's highest wind speeds.
B．It is subject to laws that restrict experimentation.
C．It is accessible to heavy machinery.
D．It is not subject to significant changes in temperature.