Where Is the Function Continuous $$f ( x , y ) = \frac { x y } { x ^ { 2 } + y ^ { 2 } }$$

Multiple Choice

Where is the function continuous? $$f ( x , y ) = \frac { x y } { x ^ { 2 } + y ^ { 2 } }$$

A) $R ^ { 2 }$
B) $x \neq 0$
C) $y \neq 0$
D) $( x , y ) \neq ( 0,0 )$