Two rods of identical dimensions, with Young's modulus $Y_{1}$ and $Y_{2}$ are joined end to end. The equivalent Young's modulus for the composite rod is
(A) $\frac{2 Y_{1} Y_{2}}{Y_{1}+Y_{2}}$
(B) $\frac{Y_{1} Y_{2}}{Y_{1}+Y_{2}}$
(C) $\frac{1}{2\left(Y_{1}+Y_{2}\right)}$
(D) $Y_{1}+Y_{2}$