If the external radius of a hollow cylinder be R units, the internal radius be r units and the height be h units, then the volume of the cylinder is
A. `pi h ( R^(2)-r^(2))` sq-units
B. `pi h (R^(2)-r^(2))` cu - units
C. `pi (R^(2)+r^(2))` cu-units
D. `pi h (R^(2)+r^(2))` cu-units
