问题如下：
An investor wants to buy a bond with the least Macaulay duration and he has three options:
Which one should he choose?
选项：
A.Bond A.
B.Bond B.
C.Bond C.
解释：
C is correct.
A bond with higher yield and shorter maturity would have less Macaulay duration if other things equal.
Macaulay duration的计算公式是怎样的？
我看书上定义是说：Macaulay duration is the weighted average number of coupon periods until a bond’s scheduled cash flows.
但是讲义的公式和助教的示例都是用的这个公式，这个不是一个 weighted 的感觉啊？
