[单选题]
A packing crate slides down an incl xk+q97;wus bbined ramp atd:/id(ytbrn). dhm8f constant velocity. Thus we can tr db y :n)i8.fd/hd(mdeduce 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.
