问题如下：
Contrary to the notion of a “zero lower bound”， negative policy rates proved to be sustainable，because
选项：
A.Trillions of dollars change hands daily: trade in goods, services, and financial instruments. These transactions cannot be accomplished by physical cash.
The move into currency did not occur as expected as the scale and speed of transactions inherent in modern economies cannot be supported using physical cash as the primary method of exchange.
Both A & B
解释：
C is correct
负利率的存在主要因为以下两点原因。
由于现代经济固有的交易规模和速度无法用实物现金作为主要的交换手段来支持，所以向货币的转变并没有如预期的那样发生。
每天有数万亿美元的交易:商品、服务和金融工具的交易。这些交易不能用现金来完成。
所以C选项入选。
意思是只有实务现金才会导致利率上下浮动？供给需求不都统计非实务现金吗？
