问题如下：
A machine manufacturer is analyzing its cost structure, and here presents the detailed information as following table shows: :
If the manufacturer increases the production units from 3 to 4, the marginal cost is equal to:
选项：
A.$35.
B.$70.
C.$85.
解释：
C is correct.
Marginal cost = change in total cost / change in quantity unit = $[(400+360)-(400+275)]/1=$85.
考点：边际成本
解析：第一步要先求总本即，它等于总固定成本+总可变成本，再套用公式可得
MC = ΔTC/ΔQ = $[(400+360)-(400+275)]/1=$85.
老师这题为什么不能用 （400+275 ）/3 - （400+360）/4 =35 这样做？
