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. Suppose you were using this rocket in outer space, far from all gravity. What would its acceleration be 3 s after fuel ignitions?

a) 30 m/s²
b) 20 m/s²
c) 10 m/s²
d) 5 m/s²