请问哪位牛牛可以给我解释一下GWD31--03-04题？ 感激不尽！
Q4:
In the year following an eight-cent increase in the federal tax on a pack of cigarettes, sales of cigarettes fell ten percent. In contrast, in the year prior to the tax increase, sales had fallen one percent. The volume of cigarette sales is therefore strongly related to the after-tax price of a pack of cigarettes.
The argument above requires which of following assumptions?
A. During the year following the tax increase, the pretax price of a pack of cigarettes did not increase by as much as it had during the year prior to the tax increase.
B. The one percent fall in cigarette sales in the year prior to tax increase was due to a smaller tax increase.
C. The pretax price of a pack of cigarettes gradually decreased throughout the year before and the year after the tax increase.
D.
For the year following the tax increase, the pretax price of a pack of cigarettes was not eight or more cents lower than it had been the previous year.
E. As the after-tax price of a pack of cigarettes rises, the pretax price also rises.
不是NN，讨论一下：
如果要证明：The volume of cigarette sales is therefore strongly related to the after-tax price of a pack of cigarettes，就必须要排除掉pretax price的变化对sales volume的影响。
pretax price在税改这一年的变化设为x, 如果 -8 cents < x <= 税改前一年pretax price的增加（如果有）， 则结论有效。如果x小于 - 8 cents，则税改在after-tax price上的效果被抵消，after-tax price维持不变或减少，销量减少有他因。如果x大于税改前一年pretax price的增加，销量的减少就有pretax price增加的影响因素。
请指正。
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