问题如下：
In the context of mortgage-backed securities, a conditional prepayment rate (CPR) of 8% means thatapproximately 8% of the outstanding mortgage pool balance at the beginning of the year is expected to be prepaid:
选项：
A.in the current month.
B.by the end of the year.
C.over the life of the mortgages.
解释：
B is correct.
CPR is an annualized rate, which indicates the percentage of the outstanding mortgage pool balance at the beginning of the year that is expected to be prepaid by the end of the year.
这道题的问题是：8%的CPR的含义？CPR是年化数据，所以就是按年计的提前偿还速度。
8%的CPR含义就是：本年提前偿还的部分是本年年初待偿本金余额的8%，这笔偿还发生在本年年末。
我突然有点困惑：提前还款速度是个啥？为啥要衡量这个速度呢？
比如这个 8%的 CPR，是本年提前偿还的部分是本年年初待偿本金余额的8%，好好的还款不行么为啥会要突然提前还款呢？
而且计算是：提前偿还的部分是本年年初待偿本金余额的8%的话，为啥不选 beginning of the year 呢？
