问题如下：
What of the following is the drawback of risk-based approach to asset classification?
选项：
A.It overestimates the portfolio diversification.
It is sensitive to historical look-back period.
It has united risk factor identification.
解释：
B is correct.
The major drawback of risk factor based approach is that it depends on the histocial data and the sepecific data period.
老师， 可以解释一下为什么这个approach会对historical look back period 很敏感？
