设X1,X2,6,X6来自总体N(0,1), =(({X)_(1)+(X)_(2)+(X)_(3))}^2+(({X)_(4)+(X)_(5)+(X)_(6))}^2 ,若CY服从x^2,则-|||-数 C= ()-|||-A. dfrac (1)(2) B. dfrac (1)(3) . C. dfrac (1)(4) . D. dfrac (1)(5)