The dipole-dipole interaction energy between stationary polar molecules and rotating polar molecules,respectively,is proportional to: [$r$ is the distance between the polar molecules]

• A
  $1 / r^3 ; 1 / r^6$
• B
  $1 / r^3 ; 1 / r^3$
• C
  $1 / r^2 ; 1 / r^6$
• D
  $1 / r^2 ; 1 / r^4$