Write an Algebraic Expression That Is Equivalent To $\sin \left( \arctan \frac { x } { 9 } \right)$

Multiple Choice

Write an algebraic expression that is equivalent to $\sin \left( \arctan \frac { x } { 9 } \right)$ .

A) $\frac { 9 } { \sqrt { x ^ { 2 } + 81 } }$
B) $\frac { 9 } { x }$
C) $\frac { \sqrt { x ^ { 2 } + 81 } } { x }$
D) $\frac { \sqrt { x ^ { 2 } + 81 } } { 9 }$
E) $\frac { x } { \sqrt { x ^ { 2 } + 81 } }$