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? ['Some of the jackmanii sold by the nursery are sold to gardeners outside North America.', 'Most North American gardeners grow clematis in their gardens.', 'The nursery sells more than ten different varieties of clematis.', 'For all nurseries in North America that specialize in clematis, at least ten percent of the clematis plants they sell are jackmanii.']