问题如下：
3 A limitation of the existing approach used by the committee to define the opportunity set is that it:
选项：
A.is difficult to communicate.
overestimates the portfolio diversification.
is sensitive to the historical look-back period.
解释：
B is correct.
A traditional approach has been used to define the opportunity set based on different macroeconomic conditions. The primary limitations of traditional approaches are that they overestimate the portfolio diversification and obscure the primary drivers of risk.
请教下老师，对于C选项这个选项本身想延伸问一下，请问sensitive to the historical look-back period的方法是哪个方法，这个是risk-based 方法的缺点吗？相关的知识点是哪个？谢谢老师。
