单项选择题
A.10~20 B.20~30 C.30~50 D.50~70
A.7 B.8 C.127 D.128
A.150.336 B.155.520 C.622.080 D.2488.320
A.600 b/s B.2400 b/s C.4800 b/s D.9600 b/s
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