A rocket burns fuel at the rate of $200 kg/s$. The exhaust gases are ejected down with a velocity of $400 m/s$. The upthrust developed by the rocket engine is

Question

$$(a)\;8 \times 10^5 N \quad (b)\;4 \times 10^5 N \quad (c)\;4 \times 10^4 N \quad(d)\;8 \times 10^4 N$$