The amplitudes and phase differences for four pairs of waves of equal wavelengths are (a) 2 mm,6 mm,...

The amplitudes and phase differences for four pairs of waves of equal wavelengths are (a) 2 mm,6 mm, and π tad; (b) 3 mm, 5 mm, and  π rad; (c) 7 mm, 9 mm, and n rad; (d) 2 mm, 2 mm, and π rad. Each pair travels in the same direction along the same string. Without written calculation, rank the four pairs according to the amplitude of their resultant wave, greatest first.