When saying that "The success of the gold-exchange standard, however, depended on the supe

rior position of the United States in world trade", the author is most probably referring that ______.

A．under this system, the United States was required to lend money to any country that asked

B．under this system, the United States was the only country allowed to maintain gold reserves

C．tying the value of most currencies to any one currency requires that the currency be Stable

D．the gold exchange standard is less flexible in meeting currency demands than the gold standard