Wavelength problem
Which sound has a shorter wavelength: 1 kHz in air or 3 kHz in water?
Wavelength = speed of sound / frequency
Air: 340 m/s / 1000 cycle/s = 0.34 m/cycle
Water: 1500 m/s / 3000 cycle/s = 0.5 m/cycle
Therefore, the answer is 1 kHz in air