Determine whether the following transactions involve spot exchange, contracts, or vertical integration.
a. A major oil company refines gasoline from crude oil produced by oil wells that it owns.
b. Transcontinental, an interstate natural-gas pipeline, has a legal obligation to purchase a specified amount of gas per week from a well owned by Fred Smith in Enid, Oklahoma.
c. A cabinetmaker purchased a dozen wood screws from the local hardware store.
d. An electric utility purchases coal from an underground mine.