If a charge q is placed at the centre of the line joining two equal charges Q such that the system is in equilibrium then the value of q is

Question

$(a)\;\large\frac{Q}{2}\qquad(b)\;-\large\frac{Q}{2}\qquad(c)\;\large\frac{Q}{4}\qquad(d)\;- \large\frac{Q}{4}$