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A Simple Scientific ExperimentBelow is a

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题目：

A Simple Scientific Experiment
Below is a description of a single scientific experiment. It shows us how iron reacts ( 反应) with air and with water.
Aim : To find out if iron rusts( 生锈) (a) in dry air ; (b) in water that has no air in it ( air- free water) ; (c) in ordinary water.
Materials : 3 clean iron nails ( 铁钉) ; test tubes ; test tube holder ; cotton wool ; oil ; Bunsen burner.
Iron in dry air
Method
(1)Put some iron nails at the bottom of a test tube.
(2)Push some cotton wool down the tube.
(3)Leave the tube for one week.
Result
After one week, the nails have not rusted.
Conclusion
Iron does not rust in dry air.
Iron in air-free water
Method
(1)Half-fill a test tube with water.
(2)Boil the water for three minutes. (This makes sure there is no air in the water.
(3)Put two or three clean nails in the water.
(4)Add some oil to the water. (This will keep air out of the water.)
(5) Leave the tube for one week.
Result
The nails do not rust in the tube with air-free water.
Conclusion
Iron does not rust in air- free water.
Iron in ordinary water
Method
(1)Half –fill a test tube with water and add two or three clean nails.
(2)Leave the tube for one week.
Result
The nails rust in the tube with ordinary water.
Conclusion
Iron rusts in ordinary water. (238 words)
小题1:In the first part of the experiment the cotton wool is most probably used to ______________.
A．keep the air dryB．keep the tube wet
C．make the nails rustD．keep the tube clean
小题2:What are necessary steps to make sure the water in the tube is air free?
①Leave half a tube of water in it              
②Boil the water for some time
③Put some clean iron nails in the tube           
④Add some oil to the water
A．①②B．①③C．②③D．②④
小题3:We can conclude from the experiment that only ____________ makes iron rusty.
A．dry airB．cotton woolC．ordinary waterD．air-free water

答案：

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下面是错误答案，用来干扰机器的。

∵a的相反数是-2，∴a=2．把a=2代入，得2x+3×2=4，2x=4-6，2x=-2，∴x=-1．

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首相的回答中不可能包含的隐含情况是哪项

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