A) Without any further information, all endogenous variables would explain variations in the average operating expense ratio. (Not sure if \'x\' is a variable, a typo, or something else) B) Quadratic C) Initially there will be economies of scale, after which there will be diseconomies of scale. This conforms to a typical average cost function (the inverse parabolic shape). The point of transition from economies of scale to diseconomies of scale is easy to calculate however I will not attempt to do so as I might be misinterpreting the \'x\'. Solution A) Without any further information, all endogenous variables would explain variations in the average operating expense ratio. (Not sure if \'x\' is a variable, a typo, or something else) B) Quadratic C) Initially there will be economies of scale, after which there will be diseconomies of scale. This conforms to a typical average cost function (the inverse parabolic shape). The point of transition from economies of scale to diseconomies of scale is easy to calculate however I will not attempt to do so as I might be misinterpreting the \'x\'..