For the signal-flow graph shown in the figure, which one of the following expressions is equal to the transfer function $\dfrac{Y(s)}{X_{2}(s)}\bigg \vert _{X_{1}(s)=0}$ ?
- $\dfrac{G_{1}}{1+G_{2}(1+G_{1})} \\$
- $\dfrac{G_{2}}{1+G_{1}(1+G_{2})} \\$
- $\dfrac{G_{1}}{1+G_{1}G_{2}} \\$
- $\dfrac{G_{2}}{1+G_{1}G_{2}}$