听力原文：M: I've just found the most useful device at that new outdoor store, a water puri

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.

(20)

A．Small and lightweight.

B．Solid and heavy.

C．Bulky but weightless.

D．Large but necessary.