题目选项分析

Last August the XT chain of gasoline stations had a temporary sales promotion in effect. In the promotion , any customer who made a purchase of ten or more gallons of gasoline was entitled to a free car wash . For the month of August , XT experienced a ten percent increase in gasoline sales as compared to sales in August the previous year so evidently the promotion was successful as a means of boosting sales.
In evaluating the argument , it would be most helpful to answer which of the following ?

【选项】In the areas in which XT's gasoline stations operate , how did total combined gasoline sales for all gasoline stations last August compare with sales for the previous August?