$5\mathrm{.}2-5.$ Dijkstra's link$-$state routing algorithm $($Part $2).$ Consider the graph shown below and the use of Dijkstra's algorithm to compute a least cost path from a to all destinations. Suppose that nodes $b$ and $d$ have already been added to $N.$ What is the path cost to the next node to be added to $N^{\text{'}}($refer to the text for an explanation of notation$).$

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$6$

$4$

$7$