If weight of rammer is W, free fall height is h, number of layers is N, number of blows is n, acceleration due to gravity is g and volume as V, then the formula for energy per volume is ______

• A. ( frac{WhNng}{V} )
• B. ( frac{WVNng}{h} )
• C. ( frac{Whng}{N} )
• D. ( frac{WgN}{hnV} )