设两个相互独立随机变量X和Y分别服从正态分布N(0,1)和N(1,1),则有( )(A)P(X+Y<=0)=0.5 (B)P(X+Y<=1)=0.5(C)P(X-Y<=0)=0.5 (D)P(X-Y<=1)=0.5