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