Why is the single$-$entity structure a defense to a Sherman Act Section $1$ violation?

Group of answer choices

Because the structure is made up of separate business entities that agree to act together.

Because as a single entity there is no agreement between two parties, as required for a Section $1$ violation.

Because there are multiple professional leagues that compete for consumers, therefore not one single entity $($league$)$ has a monopoly.

Because Congress granted the professional sport leagues an exemption under the single$-$entity structure.