Maryland enacted a statute prohibiting any producer or refiner of petroleum products from operating retail service stations within the State. The statute also required that any producer or refiner discontinue operating its company-owned retail service stations. Approximately 3,800 retail service stations in Maryland sell more than twenty different brands of gasoline. All of this gasoline is brought in from other States, as no petroleum products are produced or refined in Maryland. Only 5 percent of the total number of retailers are operated by a producer or refiner. Maryland enacted the statute because a survey conducted by the State comptroller indicated that gasoline stations operated by producers or refiners had received preferential treatment during periods of gasoline shortage. Seven major producers and refiners bring an action challenging the statute on the ground that it discriminated against interstate commerce in violation of the Commerce Clause of the U.S. Constitution. Are they correct? Explain.