Solve the Formula for the Indicated Letter $x = \sqrt { r ^ { 2 } - y ^ { 2 } }$

Multiple Choice

Solve the formula for the indicated letter. Assume that all variables represent nonnegative numbers.
- $x = \sqrt { r ^ { 2 } - y ^ { 2 } }$ for $r$

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