[单选题]
A packing crate slidpbf2 )yp +2gqkes down a6etk)d52;w nnpn 5hfq :vit z 92;arggn inclined ramp at constant velocity. Thus we can deduce thv5)ek5 qn 2ft :gi2zg9dnw6pan; th ;rat
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.
