问题如下：
Which of the following about sovereign bond is most correct for investors?
选项：
A.Floating-rate notes have least interest rate risk
B.Zero-coupon bonds have least interest rate risk
C.Fixed-coupon bonds have least interest rate risk
解释：
A is correct.
Floating-rate notes' duration is shortest (=reset date/2), thus least interest rate risk.
这个Duration是怎么计算出来的啊？为什么是一半？
