​Antitrust enforcement of vertical relationships is generally focused on

A) ​The dominant firm using horizontal contracts to extend market power to other levels of the supply chain
B) Vertical contracts reducing the intensity of competition
C) Vertical contracts that help consumers
D) ​All of the above

Antitrust enforcement of vertical relationships is primarily focused on vertical contracts reducing the intensity of competition, such as exclusive dealing, tying arrangements, and resale price maintenance. The goal is to prevent these contracts from limiting consumer choice and raising prices. While the dominant firm's use of horizontal contracts may be relevant, it is not the primary focus of antitrust enforcement in vertical relationships. Vertical contracts can potentially help consumers, but this is not the primary concern of antitrust enforcement.