A is pulling the block straight down toward the center of the Earth, no matter what the slope of the plane may be. A is the force of gravity.
The directions of B and C both depend on the slope of the plane.
B is a force that's parallel to the plane, pulling the block UP the plane. B is the force of friction.
C is a force perpendicular to the plane, preventing the block from falling down through the plane. C is the normal force.
