12(3分)设X1,X2 X3,X4是来自N(0,1)的样本, =(({X)_(1)+(X)_(2))}^2+(({X)_(3)+(X)_(4))}^2 ,当 c= () 时,cY服从 ^2-|||-A.1 B. dfrac (1)(6)-|||-C. dfrac (1)(2) D. dfrac (1)(3)