The figure shows the single line diagram of a 4-bus power network. Branches b₁, b₂, b₃, and b₄ have impedances 4z, z, 2z, and 4z per-unit (pu), respectively, where z=r+jx with r>0 and x>0. The current drawn from each load bus (marked as arrows) is equal to I pu, where I≠0. If the network is to operate with minimum loss, the branch that should be opened is _______.