A、Salt. B、Chemical pollutants. C、Germs. D、Minerals. C细节题。男士介绍净水器的功能时提到，这种净水器可以将水中的细菌完全净化出去(makes the water entirely germfree)，由此

M: I’ve just found the most useful device at the new outdoor store — a water purifier.
W: A water purifier? You want to carry even more equipment with you on our hiking trip?
M: If it’s light in weight and gave work, yes! And this one fits that description.
W: Really? I’d bet it takes up a lot of room.
M: Not at all. It’s a straw, much like the one you use to drink a soda water or a milkshake.
W: Really? That is small.
M: Yeah, it’s plastic and has a series of mineral and chemical filters that strains harmful microbes from water.
W: It’s fantastic, but can it filter the germs out of the stagnant pond water or water from lakes and streams?
M: Yes,[20]it makes the water entirely germfree.
W: But what about salt or chemical pollutants that might be in the water? Can it clean those out?
M: No, it can’t remove salt and chemicals. But those are not really major concerns when we’re out hiking and camping.
W: Mm. . . How long will a straw last?
M: For about 100 gallons, but don’t worry about measuring that large amount of water. There is a built-in safety device.[21]The mineral and chemical filters become clogged long before the limit is reached and the straw stops drawing water.
W: I think I’ll go to get one for my camping trip this weekend.
19. How does the man describe the water purifier?
20. What can the water purifier remove from the water?
21. What shows that the water purifier is going to lose effectiveness?