设总体 sim N(mu (sigma )^2) ,X1,X2,X3,X4,4,X5,X6,X7是X的样本.若-|||-.dfrac (C({X)_(1)+(X)_(2)-2(X)_(3))}(sqrt {{({X)_(4)-(X)_(5))}^2+(({X)_(6)-(X)_(7))}^2}}sim t(n) --|||-则 ()-|||-(A) n=2 =dfrac (sqrt {3)}(3) (B) n=4 =dfrac (sqrt {3)}(3)-|||-(C) n=2 =dfrac (sqrt {6)}(3) (D) n=4 =dfrac (sqrt {6)}(3)-|||-A A-|||-B B-|||-C C-|||-D D