Two unit negative charges are placed on a straight line. A positive charge q is placed exactly at the mid point between these unit charges. If the system of these three charges is in equilibrium, the value of (q in C) is

Question

$$\begin {array} {1 1} (a)\;1.0 & \quad (b)\;0.75 \\ (c)\;0.5 & \quad (d)\;0.25 \end {array}$$