1. A submarine is using ultrasound of frequency 50 kHz to measure the height of objects on the seabed. The submarine encounters object X

The phase difference between a signal from the seabed and the returned signal from object X is $\frac{2 \pi }{3}$ radians.

Assuming there is no change of phase when the signals reflect off either the seabed or the object, which of the following heights, A to D, is not a possible height h_x of object X?

The speed of sound in water is 1500 m s^–1.