问题如下：
An committee has historically used a traditional approach to define the opportunity set based on distinct macroeconomic regimes, and one analyst proposes that a risk-based approach might be a better method. Although the traditional approach is relatively powerful for its ability to handle liquidity and manager selection issues compared to a risk-based approach, there are a number of limitations associated with the existing approach. Which of the following limitation of the existing approach used by the committee to define the opportunity set?
选项：
A. is difficult to communicate.
B. overestimates the portfolio diversification.
C. 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.
什么是define the opportunity set?
