单选题 Which of the following policy tools is least frequently used by the U. S. Federal Reserve?
A. Changing the discount rate.
B. Engaging in open market operations.
C. Changing bank reserve requirements.

【正确答案】 C
【答案解析】[解析] 美联储比较常用的货币政策工具包括：调整折现率和公开市场操作；不常用的货币政策工具包括：调整存款准备金制度和说服银行加紧或者放松信贷政策。