$10\mathrm{.}3-3$ In Part $($a$)$ you will draw a network to represent the problem. The nodes of the networkrepresent time moments $($when we may make decisions$),$ as follows: Node $0$ can be "now"$($equivalently$,$ the beginning of year $1),$ node $1$ can be the end of year $1,$ node $2$ the end of year $2$and node $3$ the end of year $3.$ Nodes are connected with directed arcs $($i$,$j$),$ where i representsthe time we purchase a tractor and j the time we trade it in with a new tractor, unless it is theend of year $3$ and we do nothing. The attribute of each arc is the associated cost $($that plays therole of distance$).$ For example, there should be an arc from node $0$ to node $1$ at a cost of $8($unitsin $$1000),$ an arc from node $2$ to node $3$ at a cost of $12,$ an arc from node $1$ to node $3$ at a cost of$21,$ and so on$...$ The path from node $0$ to node $3$ with the minimum cost is associated with thebest replacement decisions during this $3-$year time horizon.