On a long horizontally moving belt,a child runs to and fro with a speed 9km `h^(-1)` (with respect to the belt) between his father and mother located 50 m apart.The belt moves with sped of 4km `h^(-1)`.For an observer on a stationary platform ,the speed of the child runing in the direction of motion of the belt is

Options

A. 4 km `h^(-1)`
B. `5 km h^(-1)`
C. 9 km `h^(-1)`
D. 13 km`h^(-1)`

Answer

D