A chemical engineer is designing the production process for a new product. The chemical reaction that produces the product may have higher or lower yield based on the temperature and stirring rate in the vessel in which the reaction takes place. The engineer decides to investigate the effect of two temperatures (50oC and 60oC) and three stirring rates (60 rpm, 90 rpm, 120 rpm) on the yield of the process. She will process three batches at each treatment combination. How many experimental units/subjects were used in this experiment