The figure shows the single line diagram of a 4-bus power network. Branches $b_{1}$, $b_{2}, b_{3}$, and $b_{4}$ have impedances $4 z, z, 2 z$, and $4 z$ per-unit (pu), respectively, where $z=r+j x$, with $r>0$ and $x>0$. The current drawn from each load bus (marked as arrows) is equal to $I \mathrm{pu}$, where $I \neq 0$. If the network is to operate with minimum loss, the branch that should be opened is
- $b_{1}$
- $b_{2}$
- $b_{3}$
- $b_{4}$