问题如下：
An annual coupon bond with a yield-to-maturity of 7% is priced at 94.75 per 100 of par value, and the coupon rate is 5%. The remaining time-to-maturity is 3 years. The Macaulay duration of this bond is:
选项：
A.2.73.
B.2.81.
C.2.86.
解释：
C is correct.
According to definition, Macaulay duration is a weighted average of the time to the receipt of cash flow, the weights are the shares of the full price corresponding to each coupon and principal payment.
The Macaulay duration of this bond is 2.86.
为什么用 effective duration 的方法做得到 Modified Duration，再乘以 （1+7%）得到的结果跟答案却不是C，正常吗？
另外，我怎么能判断出，这道题考的是 用概念来算，而不是用effective duration 方法来算？
