问题如下图：
选项：
A. 
B. 
C. 
解释：
对于a选项，从平均还款期可以理解，但是从mac.d的定义公式上来看，coupon rate越大pvcf越大，duration越大呀？还是哪里理解出错了
A bons Macaulration is inversely relateto its yielto-maturity. The Macaulration of a zero-coupon bonis less thits time-to-maturity. B is correct. A bons yielto-maturity is inversely relateto its Macaulration: The higher the yielto-maturity, the lower its Macaulration anthe lower the interest rate risk. A higher yielto-maturity creases the weighteaverage of the times to the receipt of cash flow, anthus creases the Macaulration. A bons coupon rate is inversely relateto its Macaulration: The lower the coupon, the greater the weight of the payment of principmaturity. This results in a higher Macaulration. Zero-coupon bon not pperioc coupon payments; therefore, the Macaulration of a zero-coupon bonis its time-to-maturity. coupon rate变大，每期还的钱越多，应该是正相关啊
A bons Macaulration is inversely relateto its yielto-maturity. The Macaulration of a zero-coupon bonis less thits time-to-maturity. B is correct. A bons yielto-maturity is inversely relateto its Macaulration: The higher the yielto-maturity, the lower its Macaulration anthe lower the interest rate risk. A higher yielto-maturity creases the weighteaverage of the times to the receipt of cash flow, anthus creases the Macaulration. A bons coupon rate is inversely relateto its Macaulration: The lower the coupon, the greater the weight of the payment of principmaturity. This results in a higher Macaulration. Zero-coupon bon not pperioc coupon payments; therefore, the Macaulration of a zero-coupon bonis its time-to-maturity. ration和maturity不是正相关吗？唯一的例外是时间太长的价发行债券，没有反相关的情况吧
请帮忙详细解答一下，B为什么是对的？
做对了，但是对于zero coupon的影响，想问一句zero是否看作是coupon最小的一个极端例子，所以还款最慢，ration最长。可以用这个逻辑吗？感谢解答。
