铸钢件在静止空气中30分钟内冷却到原始温差的90%。将同一件铸件在对流换热系数是静止空气5倍的流动气流中冷却到原始温差的90%所需时间是:
A. 12分钟
B. 6分钟
C. 3分钟
D. 9分钟
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关于大平板、长圆柱和圆球中一维非稳态导热问题的求解问题,下列说法,错误的是()。
A. 当Bi数较大时,可以用分析解公式进行计算
B. 当0.01 C. 当0.01 D. 当Bi数较大时,必须用诺谟图进行计算
The key point of his speech was that we should take effective measures_____water pollution.
A. stopping
B. stop
C. stopped
D. to stop
So far our company____exceptional lawn services to residential and commercial customers for 20 years.
A. will provide
B. had provided
C. provided
D. has provided
____you want some samples of our new products, you might contact our sales manager.
A. If
B. While
C. Until
D. Although