In the years following an eight-percent 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 the following assumptions?

【选项】The pretax price of a pack of cigarettes gradually decreased throughout the year before and the year after the tax increase.