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【简答题】
乙烯是一种 色, 气味的气体,密度比空气 , 溶于水,易溶于 ,可以采用 法收集。
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【单选题】有一直径为d的钢丝,绕在直径为D的圆筒上,钢丝仍处于弹性范围。此时钢丝的最大弯曲正应力 σ max = ;为了减小弯曲应力,应 钢丝的直径。
A.
σ max = Ed /( D + d ); 增大
B.
σ max = ED /( D + d ); 增大
C.
σ max = Ed /( D + d ); 减小
D.
σ max = ED /( D + d ); 减小
【单选题】The passage suggests that which of the following is a prediction concerning meteor streams that can be derived from both the conventional theories mentioned in line 18 and the new computer-derived the...
A.
Dust particles in a meteor stream will usually be distributed evenly throughout any cross section of the steam.
B.
The orbits of most meteor streams should cross the orbit of the Earth at some point and give rise to a meteor shower.
C.
Over time the distribution of dust in a meteor stream will usually become denser at the outside edges of the stream than at the center.
D.
Meteor showers caused by older by older meteor streams should be, on average, longer in duration than those caused by very young meteor streams.
E.
The individual dust particles in older meteor streams should be, on average, smaller than those that compose younger meteor streams.
【简答题】A．stems B．flows C．runs D．streams
【简答题】有一直径为d的钢丝，绕在直径为D的圆筒上，钢丝仍处于弹性范围。此时钢丝的最大弯曲正应力σ max =______；为了减小弯曲应力，应______钢丝的直径。
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