A car is traveling along the freeway at 30 m/s. The driver ... ...

  A car is traveling along the freeway at 30 m/s. The driver brakes suddenly, and the books on the front seat slide to the floorboard. It is obvious that gravity pulls the books vertically to the floorboard, but what force pulls them horizontally forward?
(A) Gravity, as well.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in 08. Momentum