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 boardequals the force of static friction. which of the following is the most general explanation for why the box accelerates down the board?

the force of kinetic friction is smaller than that of static friction, butf_gremains the same.
once the box is moving, f_gis smaller than the force of static friction but larger than the force of kinetic friction.
once the box is moving, f_gis larger than the force of static friction.
when the box is stationary, f_gequals the force of static friction, but once the box starts moving, the sliding reduces the normal force, which in turn reduces the friction.