Answer to Question 1
Some industries are more concentrated than others because of technical properties of their production technologies or unique characteristics of the markets they serve. Economies of scale, which allow firms to reduce their average costs as they increase their rates of output, favor large-scale production over small-scale production. Thus, industries for which scale economies are important (e.g., auto manufacturing and petroleum refining) are expected to be more concentrated than others in which costs do not fall as rapidly as output expands (e.g., cut-and-sew apparel manufacturing). Similarly, concentration tends to be higher in industries, such as aircraft and semiconductor manufacturing, where learning curves generate substantial production-cost savings as additional units of the original model or design are made.
Answer to Question 2
In economics, the word marginal means extra or additional. Economists reason that the optimal decision is to continue any activity up to the point where the marginal benefit equals the marginal cost, so optimal decisions are made at the point where the extra benefit received from an activity is equal to the extra cost associated with that activity.
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