A small 1 kg rocket burns fuel that exerts a time-varying upward force on the rocket. This force obeys the equation f = at². Measurements show that at t = 0, the force is 10 N, and at the end of the first 2 s, it is 15 N. Find the net force on this rocket and its acceleration 3 s after fuel ignition.

a) 54.1 N and 2.8 m/s²
b) 11.4 N and 11.4 m/s²
c) 134.1 N and 16.8 m/s²
d) 4.1 N and 12.0 m/s²