[单选题]
A packing crate slides down a)k/2,xf ztu ion inclined r 5 vh, odvh/ec0dk/fwpj0;b: iamp at constant velocity. Thus we can d0kch0h/o b:d v fid;/5 w,jepveduce 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.
