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 fiel

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 fort