I = 2p2 r vf2y02

Therefore y0 = [I/(2p2rvf2)]1/2

Density of water = 1000 kg/m3

Speed of sound in water = 1550 m/s

y0 = [(1.5X10-5 W/m2)/(2p2X1000 kg/m3X1550 m/s(60X103 Hz)2)]1/2 = 1.2X10-11 m

Answer is A

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