The correct answer is 1 only. Key Points Ultraviolet water purification is the most effective method for disinfecting bacteria from the water. Ultraviolet (UV) rays penetrate harmful pathogens in your home’s water and destroy illness-causing microorganisms by attacking their genetic core (DNA). This is extremely efficient in eliminating their ability to reproduce. ​​Disinfecting your water with Ultraviolet light is exceptionably simple, effective, and environmentally safe. UV systems destroy 99.99% of harmful microorganisms without adding chemicals or changing your water’s taste or odor. UV water purification is usually used with other forms of filtration such as reverse osmosis systems or carbon block filters. Additional Information Ultraviolet Purification Advantages Chemical Free: UV purification does not use any chemicals like chlorine or leave any harmful by-products. Taste & Odor-Free: UV does not add any chemical taste or odor to the water. Extremely Effective: One of the most effective ways to kill disease-causing microbes by destroying 99.99%. Requires very little energy: Uses about the same energy as it would run a 60 watt light bulb. Low Maintenance: Set and forget type of system, just change UV bulb annually.