In the next five years, China will import more than US$10 trillion of goods and make over US$500 billion outbound investment, and over 500 million overseas visits will be made by Chinese tourists.

问：数量单位词：hundred ,thousand,million等，要么“数词+数量单位词的单数后不用of", 要么用 "数量单位词复数+of " , 这里 US$10 trillion of goods，是这么一回事，用语法该怎么解释？