A、The water it draws changes color. B、The straw stops drawing water. C、From the built-in measuring device of it. D、By measuring

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: (23)If it’s light weight and save 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?  (23)That is small.
M: Yeah, it’s plastic and has a series of mineral and chemical filters that strains 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. (24)It makes the water entirely germfree.
W: But what about salt or chemical pollutants that might be in the water? Does it clean those out?
M: No, (24)it doesn’t remove salt and chemicals. But those are really not 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 features. (25)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. It will surely be carrying a lot of water or having to boil it at the campsite.