A、Magnesium. B、Limestone. C、Carbon. D、Water. C先看题目，再有针对性的认真听。依据“and carbon deposits began to form on the bottom of the bottle”可以

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.