In order to answer this question, we need to understand the concept of function image and the characteristics of profit function. Function image refers to the graph formed by expanding independent variables and their corresponding function values in the form of points in rectangular coordinate system. Profit function is usually expressed as: profit = selling price-cost profit = selling price? Cost. For the same commodity, the selling price and cost are fixed, so the profit function is constant. When the selling price and cost are constant, the profit function is a constant, that is, the profit is always equal to the selling price minus the cost. Therefore, even if the profit changes from 2 yuan to 1.5 yuan, the profit function is still a constant, that is, the selling price minus the cost is always equal to 1.5 yuan. So the image of profit function is still a straight line, that is, the relationship between profit and selling price is a straight line parallel to the X axis.