题目内容：

Simple Scientific Experiment（实验）

Below is a description of a single scientific experiment. It shows us how iron(铁) 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 common water. Materials: Some clean iron nails(铁钉) ; test tubes(试管); 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.	(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 common water. Conclusion Iron rusts in common water.
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.

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?

① Leave half a tube of water in it.

② Boil the water for some time.

③ Put some clean iron nails in the tube.

④ Add some oil to the water.

A. ①② B. ①③ C. ②③ D. ②④

3.We can know from the experiment that only _______ makes iron rusty.

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