A 150- Kilometer- Long Stream Has a Gradient of 10

A 150- kilometer- long stream has a gradient of 10 feet per kilometer. The streams velocity at a point X, exactly halfway between the source and the mouth, is 5 meters per second. The depth of the stream is 2 meters, and the width is 8 meters. What is the correct estimate of the stream's discharge at point X?

A) 400 cubic meters per second.
B) 600 cubic meters per second.
C) 40 cubic meters per second.
D) 80 cubic meters per second.