设随机变量X1,X 2,···,X5相互独立且都服从标准正态分布N(0,1),若-|||-.dfrac ({X)_(1)+(X)_(2)}(sqrt {{{X)_(3)}^2+({X)_(4)}^2+({X)_(5)}^2}}} 服从t分布,则 d= .-|||-A. sqrt (6)/2-|||-B. sqrt (6)/3-|||-C. sqrt (3)/3-|||-D. sqrt (3)