Why is it the case in a long-run monopolistically competitive equilibrium that the firm’s demand curve is tangent to its average cost curve? Why could it not be a long-run equilibrium if the demand curve “cut through” the average cost curve?

Why is it the case in a long-run monopolistically competitive
equilibrium that the firm’s demand curve is tangent to its average
cost curve? Why could it not be a long-run equilibrium if the demand
curve “cut through” the average cost curve