x1 = Carbon amount (% weight) x2 = Manganese amount (% weight) x3 = Chromium amount (% weight) x4 = Nickel amount (% weight) x5 = Molybdenum amount (% weight) x6 = Copper amount (% weight) x7 = Nitrogen amount (% weight) x8 = Vanadium amount (% weight) x9 = Plate thickness (millimeters) x10 = Solution treating (milliliters)

x11 = Aging temperature (degrees, Celsius)

The stepwise regression selected the following independent variables: x = Carbon, x = Manganese, x = Chromium, x = Molybdenum, x = Copper, x = Vanadium, x = Plate thickness, x = Solution treating, and x = Aging temperature. Based on this information, determine the total number of first-order models that were fit in the stepwise routine.