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

roblems 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. First 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 of 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:WeI1, 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, Rob. 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.

What is the purpose of this class discussion？

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.