If the ratio of lengths, radii and Young's Moduli of steel and brass wires in the figure are $ a $, $ b $, and $ c $ respectively, then the corresponding ratio of increase in their lengths would be:
Two charges $ -q $ each are fixed, separated by distance $ 2d $. A third charge $ q $ of mass $ m $ placed at the mid-point is displaced slightly by $ x' (x \ll d) $ perpendicular to the line joining the two fixed charges as shown in the figure. The time period of oscillation of $ q $ will be: