单项选择题
a.8%~21% b.18%~21% c.18%~20% d.18%~22%
A.上风 B.下风 C.远处 D.近处
A.管长 B.水深 C.人高 D.流速
a.4% b.0.4% c.0.5% d.1.4%
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