Economies of scale occur
a. when the marginal cost of production decreases with output.
b. when the marginal product of labor increases with output.
c. when a​ firm's long-run average costs increase with output.
d. when a​ firm's long-run average costs decrease with output.

Astudent was asked to draw a demand and supply graph to illustrate the effect on the market for premium bottled water of a fall in the price of electrolyteselectrolytes used in some brands of premium bottled​ water, holding everything else constant. she drew the graph to the right and explained it as​ follows: ​"electrolyteselectrolytes are an input to some brands of premium bottled​ water, so a fall in the price of electrolyteselectrolytes will cause the supply curve for premium bottled water to shift to the right​ (from upper s 1s1 to upper s 2 right parenthesis .s2). because this shift in the supply curve results in a lower price left parenthesis upper p 2 right parenthesis commap2, consumers will want to buy more premium bottled water and the demand curve will shift to the right​ (from upper d 1d1 to upper d 2 right parenthesis .d2). we know that more premium bottled water will be​ sold, but we​ can't be sure whether the price of premium bottled water will rise or fall. that depends on whether the supply curve or the demand curve has shifted farther to the right. i assume that the effect on supply is greater than the effect on​ demand, so i show the final equilibrium price left parenthesis upper p 3 right parenthesisp3 as being lower than the initial equilibrium price left parenthesis upper p 1 right parenthesis .p1.​" where is the flaw in the​ student's argument? supply will shift inward. demand will not shift. demand will shift inward.