# A straight rod of length 9 units slides with its ends. A and B always on the $x$ and $y$ axis respectively, then the locus of the centroid of $\Delta OAB$ is :
( A ) $x^2+y^2=9$
( B ) $x^2+y^2=1$
( C ) $x^2+y^2=3$
( D ) $x^2 + y^2=81$