假设在战后美元本位制下，外国中央银行以持有绿色美钞而非美国国库券的形式保有其美元储备，这对国际货币体系的调节机制是对称的，还是不对称的?(提示：例如，考虑当日本银行出售日元换取美元并持有这些美元时，美国和日本的货币供给会发生什么变化。) Suppose that under the postwar“dollar standard”system foreign central banks had held dollar reserves in the form of green dollar bills hidden in their vaults rather than U.S. Treasury bills．Would the international monetary adjustment mechanism have been symmetric or asymmetric? (Hint：Think about what happens to the U.S. and Japanese money supplies，for example，when the  Bank of Japan sells yen for dollar bills that it then keeps．)