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