Answer:
Economies of scale: occur when total costs for the firm go down as the firm increases output. This is why in some industries, large firms are more profitable that small firms.
Constant returns to scale: the property that occurs when increasings in factors or production (labor, capital) lead to the same increase the amount of goods or services produced.
Diseconomies of scale: this is the opposite to economies of scale. Occurs when firms experience higher costs due to larger production. They mostly occur due to coordination issues that arise when firms become to large to manage well.