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.
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.
