At a given real exchange rate,which of the following,by itself,would increase the supply of dollars in the market for foreign-currency exchange?

A.foreign citizens buy more U.S.bonds

B.U.S. citizens buy more foreign bonds

C.foreigen citizens buy more U.S. goods

D.U.S. citizens buy more foreign goods

在固定的实际汇率上，增加美元的外汇汇率，我觉得美国市民买跟多国外的债卷或者国外市民买跟多美国的产品都行？谁可以解释下这题？谢谢啦～