An organic farmer sells apples at several farmers' markets in a community. The farmer brings $25$ cases of apples to the downtown market, where they sell for $$4\mathrm{.}50$ a pound. The farmer brings $12$ cases of apples to a suburban market, where the apples sell for $$3\mathrm{.}75$ a pound. This is an example of

market fluidity.

progressive pricing.

the principle of supply.

the principle of demand.