[单选题]
A packing crate slides down an inclined ramp at5s .h6btw v7aj5j )sqd8znfs, constant 7 oae:+rv1+-ld nhzq fvelocity. Thus we can deducev7+qo1+fdne-r zh :l a that
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.
