Explain clearly

Consider the estimated regression equation y = 8.6 +2.3x1 + 3.8x2 +6.1x3, where y is the average cost of production, x1 is the cost per unit of the primary input and x2 is the level of output. As the level of output increases, ... O a. Average costs decrease at an increasing rate. O b. Average costs increase at an increasing rate. O c. Average costs decrease then increase. O d. Average costs increase then decrease