Answer:
B. The dentist can recover under an implied contract theory.
An implied contract is a type of contract that is formed based on the actions and circumstances of the parties, even if there is no written or oral agreement. In this case, the dentist performed a service for Joe by extracting his tooth, and Joe benefited from the service. This creates an implied agreement between the two parties that Joe will pay for the service. Thus, the dentist can recover under an implied contract theory.