单项选择题
A.Objects. B.Intensity. C.Rate. D.Frequency.
A.In streets. B.In airports. C.In subway stations. D.In concerts.
A.Sounds of 15,000 hertz. B.Sounds of 20,200 hertz. C.Sounds of 30,000 hertz. D.Sounds of 50,000 hertz.