Trey notices that when he increases output from Q$1$to Q$2,$ average total cost is unchanged.$$ It is likely that over this range of output, if he doubles his inputs,

a$.$ output would increase by less than $2$ times, so the production function exhibits economies of scale

b$.$ output would double, so the production function exhibits constant returns to scale. $$ Answer $\_\_\_\_$

c$.$ output would more than double, and so the production function is increasing returns to scale

d$.$ the firm must be at its minimum efficient scale.