问题如下：
When measuring a portfolio' s interest rate risk, the usefulness of portfolio duration is limited because it:
选项：
A.assumes parallel shifts in the yield curve.
B.is correct only if the portfolio's internal rate of return is equal to its cashflow yield.
C.cannot be applied if the portfolio consists of option embedded bonds.
解释：
A is correct.
Portfolio duration is the percentage change in portfolio value for a 1% change in yield, only for parallel shifts of the yield curve.
这题可否解释一下？看不懂
