If a circle of constant radius 3k passes through the origin O and meets the coordinate axes at A and B, then the locus of the centroid of triangle OAB is 

• A

  ${\mathrm{x}}^2+{\mathrm{y}}^2=(2\mathrm{k}{)}^2$

• B

  ${\mathrm{x}}^2+{\mathrm{y}}^2=(3\mathrm{k}{)}^2$

• C

  ${\mathrm{x}}^2+{\mathrm{y}}^2=(4\mathrm{k}{)}^2$

• D

  ${\mathrm{x}}^2+{\mathrm{y}}^2=(6\mathrm{k}{)}^2$