Two point charges in air exert a force of 0.36N on each other. One charge is then tripled, the other is halved, the distance between them is reduced to one-third, and the entire arrangement is immersed in oil of relative permittivity εr=4. The new force between the charges is
a) 3.24N
b) 0.405N
c) 1.22N
d) 1.62N