# If $y+3x =0$ is the equation of a chord of the circle, $x^2+y^2-30x=0$, then the equation of the circle with this chord as diameter is :
( A ) $x^2+y^2-3x+9y =0$
( B ) $x^2+y^2+3x-9y=0$
( C ) $x^2+y^2 - 3x-9y=0$
( D ) $x^2+y^2+3x+9y=0$