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 is :\\
(1) $\frac { 3 c } { 2 a b ^ { 2 } }$\\
(2) $\frac { 2 a ^ { 2 } c } { b }$\\
(3) $\frac { 3 a } { 2 b ^ { 2 } c }$\\
(4) $\frac { 2 a c } { b ^ { 2 } }$