According to text B, after adopting SODIS:

     Retrieve a discarded water bottle. Tear off the label and fill it with any water that’s not too murky from a creek, standpipe or a puddle. Place the bottle on a piece of metal in full sun. In six hours the UVA radiation will kill viruses, bacteria and parasites in the water, making it safe to drink.

     SODIS, the acronym for this Swiss - pioneered water - disinfection program, is now being used all over the world to provide drinking water for some four million people. “It’s simple, it’s free, and it’s effective,” says Ibelatha Mhelela, principal of the Ndolela Primary School in Tanzania. In 2006 her school started using SODIS to disinfect its contaminated tap water, placing bottles on the building’s corrugated metal roof. The result? Absenteeism due to diarrhea has dropped considerably, and examination scores soared. “Before we started SODIS, only ten to fifteen percent of the children passed the national sixth grade exams,” says Mhelela, “Now ninety to ninety - five percent of the students pass.” 

(National Geographic, April 2010)