Which model is used to evaluate the effects of macroeconomic policy such as tax cuts?

On a national level, the concentration ratio for grocery stores is very low. But, the "true" market for grocery stores is local in nature. What limitation does the geographic market put on the measure of concentration for grocery stores?

A) On the local level, grocery stores are more concentrated than they appear on the national level. B) On the local level, grocery stores are not as concentrated as they appear on the national level. C) The level of concentration is overestimated when viewing the national level. D) Both answers A and C are correct.

With regard to its economic profits and economic losses, how is the short run different from the long run for a perfectly competitive firm?

What will be an ideal response?