What Is the Length in Centimeters of the Base of a Rectangle

Multiple Choice

What is the length in centimeters of the base of a rectangle with diagonal of length m centimeters and height of length n centimeters?

A) $\sqrt { m ^ { 2 } + n ^ { 2 } }$
B) $\sqrt { m ^ { 2 } - n ^ { 2 } }$
C) m - n
D) $\frac { m n } { 2 }$
E) $\frac { m - n } { 2 }$