题目选项分析

Many gardeners believe that the variety of clematis vine that is most popular among gardeners in North America is jackmanii. This belief is apparently correct since, of the one million clematis plants sold per year by the largest clematis nursery in North America, ten percent are jackmanii.

Which of the following is an assumption on which the argument depends?

【选项】For all nurseries in North America that specialize in clematis, at least ten percent of the clematis plants they sell are jackmanii.