nitrobenzene has more dipole momment than nitromethane.explain???

Nitrobenzene has more dipole moment than nitro methane because in nitrobenzene there is greater charge separation due to the resonance.

As we know, dipole moment is directly proportional to the charge and distance . D = d $\times$ e 
where  d = distance of separation and e = charge.
In nitro methane there is no such kind of charge separation. Therefore, it has less dipole moment.