听力原文：S: Let's talk about the results of your laboratory？ experiment. Did you have any

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.

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A．To discuss the results of the lab experiment.

B．To answer Bob' s question about the lab experiment.

C．To explain the method of collection by water displacement.

D．To prepare the students to do the lab experiment.