听力原文：S: Let's talk about the results of your laboratory? experiment. Did you have any problems with it? B: Yes, Professor Smith. We did. S: Who's your lab partner, Bob? B: Anne Wilson. S: Well, Anne, can you and Bob go over the procedure for the class? A: Sure. Firs( we put ten grams of crushed limestone in a bottle. S: Anything special about the bottle? B: It was a gas - collecting bottle with a one - hole stopper and bent glass tubing. S: Very good. So you put the limestone in a gas - collecting bottle. Then what? A: Then we poured in ten milligrams of hydrochloric acid, put on the stopper, and collected a bottle of carbon dioxide. S: Right, What was the method of collection? A: Water displacement. S: Good. A: Then, we lit a magnesium ribbon and put it in the bottle or carbon dioxide. B: And carbon deposits began to form. on the bottom of the bottle, You see, we didn't have any problem with procedure. A: Well, we had a little problem getting the magnesium ribbon to stay lit until we could get it into the bottle. B: Okay. But we did it. The big problem was that we really didn't understand what happened. Did the magnesium combine with the oxygen in the carbon dioxide? S: You have just answered your own question, Bob. The burning magnesium broke the carbon - oxygen bonds in the carbon dioxide, and then the oxygen combined with the magnesium to produce magnesium oxide. A: And the carbon was freed to deposit itself on the bottle. S: Exactly. (23)