Consider three individuals, John, Mark, and Abigail. John and Mark live in single$-$family homes

and use $6$ CCF$/$month and $9$ CCF$/$month$,$ respectively. $($CCF $=$ centum cubic foot, i$.$e$.,100$ cubic

feet.$)$ Abigail lives in an apartment building with a state$-$of$-$the$-$art greywater recycling system

and uses $2$ CCF$/$month$.$

a$.$ Last month, Abigail$$s bill was $$4,$ John$$s was $$12,$ and Mark$$s was $$18.$ What type of

pricing structure is the utility using?

b$.$ The utility has changed pricing structures. This month Abigail paid $$10,$ and John paid

$$12,$ and Mark paid $$12.$ What type of pricing structure is the utility using now?

c$.$ The utility has changed pricing structures again. This month Abigail paid $$2\mathrm{.}50,$ John

paid $$12,$ and Mark paid $$19\mathrm{.}50.$ What type of pricing structure is this?