Two thin wire rings each having radius R are placed at a distance d apart with their axes coinciding . The charges on the two rings are +Q and -Q .The potential difference between the centres of the two rings is

Question

$(a)\;zero\qquad(b)\;\large\frac{kQ}{d^2}\qquad(c)\;2kQ\;(\large\frac{1}{R}-\large\frac{1}{\sqrt{R^2+d^2}})\qquad(d)\;kQ(\large\frac{1}{R}-\large\frac{1}{\sqrt{R^2+d^2}})$