A、A book full of knowledge. B、A special book that purifies water. C、A new water fountain. D、Clean and drinkable water. B新闻中提到，研究

The expression "a thirst for knowledge" may soon have a new meaning for millions of people who have no way to get clean water.
     (5)Researchers have developed a book with specially treated pages that can make water safe to drink. The researchers say their invention could improve the lives of many in the developing world. The special book could turn dirty water into clean, drinkable water. The book contains 25 pages. Each page is about one millimeter thick.(6)The pages contain very small particles of silver. The pages can be used as filters to remove harmful microorganisms that can pollute drinking water. The filter kills the organisms that pass through it.(7)Pictures on the pages show the dangers of dirty water and how to use the book for those unable to read. The pages are made of filter paper. They are designed to be torn from the book. Water can be poured through the paper to be decontaminated. Researchers tested the drinkable book in Bangladesh, Ghana and South Africa. The tests proved to be successful.
5. What is the researchers’ new invention?
6. Why can the pages be used as filters?
7. What do we learn about the pages?