Homework Help: Find force on a crate while a driver accelerates

1. The problem statement, all variables and given/known data
A crate with a mass of 250 kg rests on the bed of a truck that is moving at a speed of 40.0 km/h. The driver accelerates to a speed of 70.0 km/h in 8 s. Assuming the force is constant, what force acts on the crate during this time? Assume that the crate does not slide on the bed of the truck.
A) 18 N
B) 3400 N
C) 240 N
D) 260 N


2. Relevant equations

f=ma

3. The attempt at a solution
Well I would think that you would find acceleration giving you 3.8m/s2 and then take 250/3.8 but that gets you 65.789 which is not an option.

 

That helped. Thanks!