[单选题]
A packing crate slidf8n *99 z2lizv t7wf cb:dbex7es do ad0t33;txeb,x-yma bwn an inclined ramp at constant velocity. Thus we can dedtb03;-xmat x3e a, byduce 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.