A rocket is flying straight up at constant velocity. The rocket's upward force is 4×10⁵ N, and its mass is 1×10⁶ kg. Using Newton's 2nd law, what is the net force acting on the rocket?

a) 4×10⁵ N
b) 0 N
c) 1×10⁶ N
d) 5×10⁶ N