An oil company wishes to study the effects of four different gasoline additives on mean gasoline mileage. The company randomly selects four groups of six automobiles each and assigns a group of six automobiles to each additive type (W, X, Y, and Z). Here all 24 automobiles employed in the experiment are the same make and model. Each of the six automobiles assigned to a gasoline additive is test driven using the appropriate additive, and the gasoline mileage for the test drive is recorded. What are the treatments?