A．Wearing arm weights while you are swimming.
B．Jogging vigorously in one place for a long time.
C．Using bicycles that require you to use both your arms and legs.
D．Walking slowly while swinging your arms back and forth.

A．They use a map and the memory of location.
B．They use the sun and the stars.
C．They use a map and the earth’s magnetic field.
D．They have precise sense of time or may use the earth’s magnetic field.