Solve the Equation for the Unknown $x ^ { 2 } = 81$

Solve the equation for the unknown.
- $x ^ { 2 } = 81$

A) $x = 9$
B) $x = \pm 9$
C) $x = \frac { 81 } { 2 }$
D) $x = \pm \frac { 81 } { 2 }$