A new business, CoolIT, sells items to keep you cool when outside. Its products include coolers, generators, floats, and other water accessories designed to beat the heat. One of its most popular products, a solar cooler, was redesigned after two years of being on the market. CoolIT would like to know if monthly sales were higher for the redesigned model. CoolIT has 24 months of sales for the original model and 24 months of sales for the redesigned model. What would be the appropriate test statistic to see if the average monthly sales were different?