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