2.设X与Y相互独立,且 approx N(1,9),Yapprox N(2,4) 则 =x-y 仍是正态分布,且有 __-|||-(A) backsim N(-1.5) ;(B) backsim N(-1,13); (C) approx N(3,36); (D)-|||-backsim N(3,13)