问题如下：
The holding period for a bond at which the coupon reinvestment risk offsets the market price risk is best approximated by:
选项：
A.duration gap.
B.modified duration.
C.Macaulay duration.
解释：
C is correct.
When the holder of a bond experiences a one-time parallel shift in the yield curve, the Macaulay duration statistic identifies the number of years necessary to hold the bond so that the losses (or gains) from coupon reinvestment offset the gains (or losses) from market price changes. The duration gap is the difference between the Macaulay duration and the investment horizon. Modified duration approximates the percentage price change of a bond given a change in its yield-to-maturity.
当gap=0的时候，不就是相等的时候吗，为什么不选A
