Steve

 YTM就是 required return 用来discount target future value to present.
假设yield curve flat, which means reinvestment return = YTM 所以这个时候你只要能够使immunization target rate = YTM = reinvestment rate就行了

但是，若yield curve downward sloping的话，你就有reinvestment risk，因为你在hedging period 中的income (coupon payment)就会被reinvested at a lower rate than when the hedge was initiated，所以这个时候就要求(intuitively)你的immunization rate > YTM 去compensate这个reinvestment loss

Steve

 看了一下，题目上说b/c of higher reinvestment return...好像和我解释的相反。。我觉得应该是lower IR