21.下圖 RLC 電路,其共振頻率 f o 及共振之功率因數 PF 分別為 (A) f o =1 / (2π RC),PF = 0(B) f o =1 / (2π RC),PF =1 (C) f o =1 / (2π LC),PF = 1 (D) f o =1 / (2π RC),PF = 1