Three pairs of objects move along the same straight line. In the ground frame their velocities are as follows:
Pair I: object 1 at +4 m/s, object 2 at –2 m/s
Pair II: object 1 at +3 m/s, object 2 at +7 m/s
Pair III: object 1 at –5 m/s, object 2 at +1 m/s
Identify every pair for which the magnitude of the relative speed between the two objects is the greatest among the three pairs. State the pair number(s).