A、Guide and tourist. B、Salesman and customers. C、Teacher and student. D、Engineer and customer. C对话中两个说话人的关系不是特别明显，从男士的第一句话来看，A项

M: Now that you are all here, let me show you around this plant, and if you listen closely, you might learn something about glass and crystal.
W: I had nothing about the structure of crystal before I have just learned in our science class. I always thought there was crystal in glass.
M: Well, real crystal is generally a hard substance in which the atoms are arranged in a highly ordered structure much like the framework of a skyscraper.
W: Does that mean in ordinary glass the molecules are not ordered?
M: That’s right, but most of the glass made here is actually lead crystal or glass with a high lead content.
W: Lead? Isn’ t lead poisonous?
M: What it does is to make a very good quality glass. You will see several samples of glass on the tour. If everyone is ready now, please follow me.
W: I am supposed to hand in a paper in this aspect after the tour.