What is one reason chlorination would be preferred over ultraviolet radiation? a) chlorine only eliminates visible impurities and is ineffective against microorganisms. b) chlorine not only kills bacteria but provides a residual effect on returning microorganisms. c) uv radiation can be used only to treat drinking water and is ineffective in treating waste water. d) ultraviolet radiation is more effective because it reduces the ph of water and destroys all bacteria.