Widgets R Us uses a variety of different items to produce its world$-$class widgets. Classify each item according to whether it generates an explicit or implicit cost for the firm.

natural gas from the gas utility

production$-$line workers

the current rental value of the land Widgets R Us bought $50$ years ago for $$10$

quality$-$control$-$testing machines obtained from an independent supplier

the purchase of a used car

warehouses rented to store inventory

consulting services that the managers of Widgets R Us think are necessary

trucks owned outright by Widgets R Us