听力原文：M: Want to go out and get something to eat? W: I can't. I have a chemistry midterm on Monday and a German exam on Tuesday. M: I have a geology exam Monday myself, but I think I'm ready for it. W: What kind of exam is it going to be—multiple choice or essay? M: Neither. The professor is going to give us a mineral sample and we have to identify it. W: How do you do that? I mean, a rock's a rock, isn't it? M: Actually, there are a lot of tests you can perform. on minerals to help you figure out what they are. Probably the first tests I'll do are scratch tests. When you do a scratch test, you rub the sample on a known mineral to see if the unknown mineral scratches the known mineral or vice versa. That tells you the relative hardness of the sample. W: What other tests will you do? M: I'll probably do a streak test next. In that test, you rub an unknown mineral against a piece of china to see what color the streak is. W: Why can't you just look at the mineral to see what color it is? M: Well, you can, but sometimes a mineral has some other substance in it, and the substance can change its color. But a streak test shows the mineral's true color. Then there's always the specific gravity test, the blowpipe test...oh, and... W: And after you've done all these tests, you can positively identify any mineral? M: Well, usually...but not always. I just hope I can on Monday! (20)