A girl shoves a 4-kg toy cart across the level floor with a ... ...

  A girl shoves a 4-kg toy cart across the level floor with a speed of 15 m/s (so it is going 15 m/s when it leaves her hand). It slides to a rest in 5 s. Assuming a constant force slowing the cart, what is the magnitude of the force?
(A) 0.75 N
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in 03. Laws of Motion