Find Two Functions Defined Implicitly by the Given Relation $$x ^ { 2 } + y ^ { 2 } = 81$$

Multiple Choice

Find two functions defined implicitly by the given relation.
- $$x ^ { 2 } + y ^ { 2 } = 81$$

A) $y = \sqrt { 81 } + x$ or $y = \sqrt { 81 } - x$
B) $y = \sqrt { 81 - x ^ { 2 } }$ or $y = \sqrt { 81 + x ^ { 2 } }$
C) $y = \sqrt { 81 } + x$ or $y = - \sqrt { 81 } - x$
D) $y = \sqrt { 81 - x ^ { 2 } }$ or $y = - \sqrt { 81 - x ^ { 2 } }$