When a board with a box on it is slowly tilted to larger and larger angle, common experience shows that the box will at some point "break loose" and start to accelerate down the board. the box begins to slide once the component of gravity acting parallel to the board fg just begins to exceeds the maximum force of static friction. which of the following is the most general explanation for why the box accelerates down the board?

a. the force of kinetic friction is smaller than that of static friction, but fg remains the same.
b. once the box is moving, fg is smaller than the force of static friction but larger than the force of kinetic friction.
c. once the box is moving, fg is larger than the force of static friction.
d. when the box is stationary, fg equals the force of static friction, but once the box starts moving, the sliding reduces the normal force, which in turn reduces the friction.