[单选题]
A packing crate slides down an inclined.5k ui*fucbb/ ramp at constantm/l0;b b+llxd velocity. Thus we can deduce tmbbx /d;+l ll0hat
A. a frictional force is acting on it.
B. a net downward force is acting on it.
C. it may be accelerating.
D. it is not acted on by appreciable gravitational force.
