9.设x1,x2,x3是来自总体X的样本,若 (X)=u (未知), hat (mu )=dfrac (1)(2)(x)_(1)-a(x)_(2)+3a(x)_(3) 是μ的-|||-无偏估计,则常数 a=-|||-A. dfrac (1)(6) B. dfrac (1)(4) C. dfrac (1)(3) D. dfrac (1)(2)