If the efficiency of a lifting machine is kept constant, its velocity ratio is __________ propotional to its mechanical advantage.

A. if a system of coplanar forces is in equilibrium, then their algebraic sum is zero
B. if a system of coplanar forces is in equilibrium, then the algebraic sum of their moments about any point in their plane is zero
C. the algebraic sum of the moments of any two forces about any point is equal to moment of theiwesultant about the same point
D. positive and negative couples can be balanced
E. none of the above