听力题 M: I''ve just found the most useful device at that new outdoor store, a water purifier. W: A water purifier! You want to carry even more equipment with you on our hiking trips? M: If it is lightweight and saves work—yes! And this one fits that description. W: Really? I bet it takes up a lot of room. M: Not at all. It''s a straw, much like the ones you use to drink a soda or a milkshake. W: Really? That''s small. M: It''s plastic and has a series of mineral and chemical filters that strain harmful microbes from water. W: Yes, but how good a job can it do filtering the germs out of stagnant pond water, or water from lakes and streams? M: It''s fantastic. It makes the water entirely germ-free. W: But what about salt or chemical pollutants? Does it clean them out? M: No, it doesn''t remove salt and chemicals. But these are really not major concerns when we''re out hiking and camping. W: Hm... 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 feature. The mineral and chemical filters become clogged long before the limit is reached, and the straw then stops drawing water. W: I think I''ll go and get one for my camping trip this weekend.