Which of the following best represents a two$-$part tariff?

a$.$ bulk$-$discount pricing offered by Lowe's: buy $50$ pieces of $24$ lumber at $$1$ eacti and the next $50$ pieces at $75$ cents each

b$.$ paying for a Gillette razor, then buying a pack of five razor blades

c$.$ haggling with a used car salesman in negotiating the price of a new sedan

d$.$ paying a $$2$ toll to drive on the parkway during the week, but paying a $$4$ toll for the same parkway during th weekend