20．已知样本$p_{1}:ax_{1}$，$ax_{2}$，$\ldots$，$ax_{n}$的均值为4，标准差为$m$，样本$p_{2}:2x_{1}-1$，$2x_{2}-1$，$\ldots$，$2x_{n}-1$的均值为3，方差为4，则下列结论正确的是$($　　$)$