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【单选题】如图示供水管线。管长 L, 流量, 今因检修管子, 用若干根直径为0.5d、管长相同于L的管子并联代替原管, 保证输水量不变, 设λ为常数,ε/d相同，局部阻力均忽略，则并联管数至少需要 根。
A.
3
B.
6
C.
4
D.
5
【单选题】The Grand Canyon Standing on the rim of the Grand Canyon(大峡谷), gazing across this giant wound in the Earth’’s surface, a visitor might assume that the canyon had been caused by some ancient convulsion...
A.
it was caused by some ancient convulsion
B.
its presence is an embarrassment to the state of Colorado
C.
it looks like an injury on the Earth’s surface
D.
it has caused many visitors to injure themselves