问题如下：
Andy is a businessman selling socks in the local market. If 100 pairs of socks are sold, he can earn a total revenue of $500. If 120 pairs of socks are sold, the total revenue will be $590. The marginal revenue of socks with a market 120 pairs of socks is:
选项：
A.$4.5
B.$5.0
C.$4.9
解释：
A is correct.
Marginal revenue is the increase in total revenue from selling one more unit of a good. In this case, selling 20 more pairs of socks increase the revenue of $90 ($590-$500), therefore the marginal revenue is $90/20=$4.5.
考点：边际收益
解析：总收益= $90 =$590-$500
边际收益=总收益的增加÷产量的增加
那么直接套用公式90/20=$4.5.
with a market 120 pairs of socks 怎么理解
