A.5~10M B.1~5M C.5~10k D.1~5k
A.15°~20° B.20°~25° C.20°~30° D.30°~35°
A.0.693C2R1R2/(R1+R2) B.0.693(R1+R2)/C2R1R2 C.0.693R2C2 D.0.693R1C1