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【单选题】
流体在圆形水平直流动，测得平均流速为0.5 m/s，Re =1000，压力降为15Pa。若将流速增加一倍，其他条件不变，则压强降变为（ ）。A.
15 Pa
B.
30 Pa
C.
20 Pa
D.
60 Pa
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皮皮学刷刷变学霸
举一反三
【单选题】12 28. Since the beginning of 2013,these regulations ____________ across the country.
A.
will be introduced
B.
have been introduced
C.
are introduced
D.
were introduced
【单选题】Since the beginning of 2013, this regulations _________ in most workplaces across the country. (2016.12)
A.
will be introduced
B.
have been introduced
C.
are introduced
D.
were introduced
【简答题】since the 10th century, the population of Paris ________
【单选题】Since the beginning of 2013, these regulations in most workplaces across the country. (2016.12)
A.
will be introduced
B.
are introduced
C.
have been introduced
D.
were introduced
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