单项选择题
A.5B.10C.12D.15
A.47.5Hz—51.5HzB.48.5Hz—51.5HzC.49.5Hz—50.5HzD.47.5Hz—52.5Hz
A.-15%—+10%B.-10%—+10%C.-10%—+15%D.-15%—+15%