问题如下：
All of the following items are generally considered advantages of non-parametric estimation methods except:
选项：
A. ability to accommodate skewed data.
B. availability of data.
C. use of historical data.
D. little or no reliance on covariance matrices
解释：
The use of historical data in non-parametric analysis is a disadvantage, not an advantage. If the estimation period was quiet (volatile) then the estimated risk measures may understate (overstate) the current risk level. Generally, the largest VaR cannot exceed the largest loss in the historical period. On the other hand, the remaining choices are all considered advantages of non-parametric methods. For instance, the non-parametric nature of the analysis can accommodate skewed data, data points are readily available, and there is no requirement for estimates or covariance matrices.
选项D应该怎么理解啊？
