Some spherical equipotential surfaces are shown in figure. The values of the potentials are 100 V, 80 V, 40 V on surfaces of radii 10 cm,12.5 cm and 25 cm. The electric field at a distance r from the common centre is
• A
${{20} \over {{r^2}}}$
• B
${{10} \over {{r^3}}}$
• C
${{20} \over {{r^3}}}$
• D
${{10} \over {{r^2}}}$