Fill in the Blanks (Reading & Writing)

Public swimming pools must balance hygiene with cost, since chlorine levels have to be carefully —monitoredmonitormonitoringmonitors throughout the day. Too little disinfectant allows bacteria to —thrivethrivedthrivingthrival, while too much can —irritateirritatedirritatingirritation swimmers' eyes and skin. Regular water testing therefore remains —essentialessentiallyessenceessentials for any facility open to the public.