Cubical Blocks of Mass M and Side L Are Piled $\frac { 5 } { 2 }$

Multiple Choice

Cubical blocks of mass m and side l are piled up in a vertical column.The total gravitational potential energy of a column of three blocks is

A) $\frac { 5 } { 2 }$ mgl.
B) 3mgl.
C) $\frac { 9 } { 2 }$ mgl.
D) 6mgl.
E) 9mgl.