Free ( foreign) exchange: Foreign exchange which is freely convertible. 自由外汇:可自由兑换的外汇。
External causes is preferences and the promotion of the ruling class, the free and open society and the great fusion of Chinese and foreign cultural exchange. 外因是统治阶级的喜好和提倡、唐代自由开放的社会风气以及境内各民族大融合与中外文化交流。