You Are Standing in a Moving Bus,facing Forward,when You Suddenly

Multiple Choice

You are standing in a moving bus,facing forward,when you suddenly slide forward as the bus comes to an immediate stop.What force caused you to slide forward from the point of view of the observer on the ground?

A) the force that Earth exerts on you
B) the force exerted by the floor of the bus on you perpendicular to the floor
C) the force exerted by the floor of the bus parallel to the floor (the friction force)
D) There is not a force causing you to your fall.