Problem $8-12($Set Covering Method$)$

A volunteer fire department is planning to serve the rural communities shown in network below. It has just purchased two used fire engine auctioned off by a nearby city. The travel time in minutes between communities in the service area is shown on the network below. The fire department is interested in determining the number and location of two fire engines to serve the communities so that a fire engine will be within $30$ minutes' travel time of any community.

a$)$ Select all possible pairs of communities in which the two fire engines could be located to ensure that all communities can be reached in $30$ minutes or less.

b$)$ What additional consideration could be used to make the final site selection from the community pairs found in part $($a$)?$

c$)$ What is the final site selection from the community pairs found in part $($a$)?$