If You Are Sailing Across the Continental Shelf in Queen

If you are sailing across the continental shelf in Queen Charlotte Sound and your echo sounder reported the bottom at 200 milliseconds (0.200 sec) at the maximum depth crossing the axis of Mitchell Trough, how deep is the seabed in the middle of this drowned submarine valley? Use the equation: depth (m) = 0.5 * (1500 m/s * total 2-way travel time in sec)

A) 150 metres
B) 15 metres
C) 1500 metres
D) 500 metres