The earliest international monetary system was known as ______ u

The earliest international monetary system was known as ______ under which countries pledged to change their paper currencies into gold when requested to do so.
A、the gold standard
B、the Bretton Woods System
C、the flexible exchange rate system
D、the indirect exchange rate
【正确答案】：A
【题目解析】：The earliest international monetary system was known as the gold standard under which countries pledged to change their paper currencies into gold when requested to do so.
参见教材P226。