In the long run, firms will expand as long as there are more , and new firms will enter the industry as long as they earn .

Economies of scale; positive economic profits.

Explanation:

Economies of scale directly or imply increase or massive production by a company or group of firm. And in most cases this is not seen to be collective as it is unique and implies to a particular firm in their cause of business. This make shift can be caused or triggered by a lot of factors; example can be seen when things are been done manually with man power and at the long run a machine is been put to place, causing massive production.

Positive economic profit on the other hand is simply seen to be the effect of the economies of scale as it causes growth in the company, sometimes pushing to opening of many other branches of the same company. It is also seen to benefit the buyers as unit per capital of the said goods will have to reduce in cost too.

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the first successful, urban blues recording, selling more than 75,000 copies, was made in year 1920.

mamie smith recorded the first commercially successful example of the urban blues,   with the recording of “crazy blues”."crazy blues" was recorded on august 10, 1920, in new york city. within a month of the release of “crazy blues” it had sold 75,000 copies.