Determine the minimal spanning tree of the network of Example 6.2-1 under each of the following separate conditions:

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(a) Nodes 5 and 6 are linked by a 2-mile cable.

(b) Nodes 2 and 5 cannot be linked.

(c) Nodes 2 and 6 are linked by a 4-mile cable.

(d) The cable between nodes 1 and 2 is 8 miles long.

(e) Nodes 3 and 5 are linked by a 2-mile cable.

(f) Node 2 cannot be linked directly to nodes 3 and 5.