[单选题]
A packing crate slides xgj/aj.izfjj(tepu3,p sd. ,3 w49tn down an inclined ram;lo7 ,ge1qnmchzw ,j1p at constant velocity. Thus we can deduwlhj, nc;qegm17, o1z ce 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.
