[题目]一长为l的均匀细棒悬于通过其一端的光-|||-滑水平固定轴上,如图所示,作成一复摆。已知细-|||-棒绕通过其一端的轴的转动惯量 =dfrac (1)(3)m(L)^2, 此摆作-|||-微小振动的周期为 ()-|||-l-|||-A. pi sqrt (dfrac {1)(9)}-|||-B. pi sqrt (dfrac {1)(29)}-|||-C. pi sqrt (dfrac {21)(39)}-|||-D. pi sqrt (dfrac {1)(39)}