An international medium of exchange is required for international trade. From the late 1800s until World War Ⅰ, most countries operated on the gold standard. Gold coins of standard specifications circulated freely between countries, making gold in effect an international currency. This system provided an automatic correction for some trade imbalances, but it had little liquidity (the money supply could not expand as rapidly as required by expanding trade), and it was vulnerable to short-term changes in the gold supply.
After the financial instability of the 1930s, the international monetary(货币的) system was rebuilt following World War Ⅱ on the gold-exchange standard. The values of most national currencies were fixed in relation to the U. S. dollar; reserves were kept in dollars, which could be exchanged on demand for gold at a set price (＄35 an ounce until 1968). The International Monetary Fund (IMF), a key institution set up under this system, makes international loans with capital subscribed by its members which include most noncommunist states. Voting rights are proportional to the amounts subscribed. The IMF has been able, through its loans, to stabilize fluctuating currencies and to influence the internal financial policies of recipient(接受的) countries, a frequently criticized practice.
The success of the gold-exchange standard, however, depended on the superior position of the United States in world trade. In the 1960s, continual balance of payments deficits(赤字) lowered U. S. gold re serves and fatally undermined the system. In 1968 a two-tiered(两极的) system was adopted. Government banks maintained fixed gold prices, while nongovernmental buyers traded freely. Simultaneously, non-dollar special drawing rights (SDRs) were assigned to IMF members in proportion to their contributions. But these changes did not relieve strain on the U. S. dollar. In 1971 President Richard Nixon announced that dollars would no longer automatically be exchanged for gold, and since then there has been no single international monetary standard.
As a measure of money flow, the balance of payments differs from the balance of trade primarily because of its greater ______.

A. specificity
B. accuracy
C. ability to predict future trends
D. comprehensiveness