a b c d

b

Force applied on the surface is $ F = \frac{P}{c} = \frac{IA}{c}$ $\Rightarrow \text{Momentum delivered } = Ft = \frac{Pt}{c} = \frac{IAt}{c} = \frac{20\times 30\times 30\times60}{3\times 10^8} = 36\times 10^{-4} kg m/s$