题目内容：

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)

1.In the first part of the experiment the cotton wool is most probably used to ______________.

   A. keep the air dry                            B. keep the tube wet

   C. make the nails rust                          D. keep the tube clean

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. ①②                  B. ①③                 C. ②③                D. ②④

3.We can conclude from the experiment that only ____________ makes iron rusty.

A. dry air       B. cotton wool           C. ordinary water         D. air-free water