Multinational oil companies such as ExxonMobil, Royal Dutch Shell, or BP are active all the way along the supply chain from locating crude oil deposits, drilling and extracting crude, transporting it around the world, refining it into petrochemicals such as gasoline, to distributing the fuel to company-owned retail stations, where it is sold to consumers. To gain this complete control of the various resources needed to product oil and gas, the companies had to have engaged in _____ mergers.