Final answer:
True, to prove a violation of the Sherman Act, evidence must show the defendant had monopoly power and willfully maintained it, often through anticompetitive practices. Therefore, the given statement is True.
Step-by-step explanation:
To prove a defendant had a monopoly that violates the Sherman Act, it is true that there must be evidence the defendant had monopoly power and willfully acquired or maintained that power. Under U.S. antitrust laws, having a monopoly isn't illegal per se; it depends on how that monopoly was acquired or maintained.
For example, if a firm has a monopoly due to a newly patented invention, this is permissible as it is seen as a reward for innovation. However, if a firm has willfully acquired or maintained monopoly power through restrictive practices, such as exclusionary or anticompetitive behavior, then it may be in violation of the Sherman Act.