Now think of another product in American history that doubled every decade for four decades, and then imagine that that product became the country's, without question, absolute largest export.

思考下在美国历史中有没有另一种产品曾,有如此辉煌的成就,那么这个产品将毫无疑问地,成为美国最大的出口产品

耶鲁公开课 - 美国内战与重建课程节选