[单选题]
A packing crate slides down an inclined ramp at constant s(c . 3m6oliisy4 pxy1velocity. Thux ;)zvn 1vcafzwa/8p7s we can d;z8vap7)n zx v/w1faceduce 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.