Find the Domain of the Function $f ( y ) = \sqrt { 9 - y ^ { 2 } }$

Find the domain of the function.
$f ( y ) = \sqrt { 9 - y ^ { 2 } }$

A) $- 3 \leq y \leq 3$
B) $y \leq - 3$ or $y \geq 3$
C) $y \geq 0$
D) $y \leq 3$
E) all real numbers