Which of the following are arranged in decreasing order of dipole moment?

The dipole moment of  ${\mathrm{CH}}_3\mathrm{Cl}$, ${\mathrm{CH}}_3\mathrm{Br}$ and ${\mathrm{CH}}_3\mathrm{I}$ decreases as the electro negativity of the halogen decreases. However, due to much smaller size of F as compared to that of CI, dipole moment (product of charge and distance) of ${\mathrm{CH}}_3\mathrm{F}$ is lower than that of ${\mathrm{CH}}_3\mathrm{CI}$ . Thus option (b) is correct.