问题如下：
Profit maximizng output under perfect competition will not occur when:
选项：
A.the difference between total revenue and total costs is the smallest.
B.marginal revenue is equal to marginal cost.
C.the revenue value from the last unit generates equals the cost of producing that unit.
解释：
A is correct.
The difference between total revenue and total cost should be maximized, then profit maximizng output under perfect competition will occur.
考点：利润最大化
解析：
收入和成本差异就是利润。这个差异越小就代表利润越小，A选项入选。
MR=MC是利润最大化的条件。MR>MC，扩大上产可以提高利润。相反MR＜MC，减少生产可以提高利润。B选项不入选.
C的说法与B是一个意思，所以也不入选。
相反MR＜MC，减少生产可以提高利润？？？
