题目3.设随机变量X,Y相互独立,且X~N(3,9),Y~N(-1,4),则 Z=X-Y~( )A. N(2,13)B. N(2,5)C. N(4,13)D. N(4,5)3.设随机变量X,Y相互独立,且X~N(3,9),Y~N(-1,4),则 Z=X-Y~( )A. N(2,13)B. N(2,5)C. N(4,13)D. N(4,5)题目解答答案C. N(4,13)