gallons of milk. Plastic Company dominates the plastic container market for holding halfgallon and gallons of milk. One advantage of paper cartons is that they are more easily and more cheaply recycled. Competition is fierce between the two companies. Congress passed a law last month that assesses a five$-$cent per container tax on plastic containers. The author of the bill stated a desire to encourage consumers to use more "Earth friendly" containers. Plastic company believes they are not being treating equally and this action of Congress is unconstitutional. Which of the following tests would a court use in determining whether the legislation is constitutional?