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

- $\frac{G_{1}}{1+G_{2}(1+G_{1})}$
- $\frac{G_{2}}{1+G_{1}(1+G_{2})}$
- $\frac{G_{1}}{1+G_{1}G_{2}}$
- $\frac{G_{2}}{1+G_{1}G_{2}}$