单项选择题
A.5天 B.7天 C.15天 D.30天
A.48.5〜50.5Hz B.49.5〜50.5Hz C.49.0〜51.0Hz D.49.5〜51.5Hz
A.±0.033Hz B.±0.05Hz C.±2r/min D.±3r/min