You have two populations. One has a mean of 4 and a variance (方差)of 2. The other has a mean of 5 and a variance of 4. You randomly draw one number from each of the populations and compute the difference value(从两总体中随机各选出一个数并计算出差值). If you did this over and over again (重复足够多次), what would be the variance (方差)of the differences?