<prompt> Which of the following is the primary mechanism of action of fluoride on topical application?Forming a reservoir in salivaInhibition of plaque bacteriaImprovement in tooth morphologyConversion of hydroxyapatite to fluorapatite by replacing the OH- ions  <response>: The presence of fluoride reduces the solubility of enamel by promoting the precipitation of hydroxyapatite and phosphate mineral. Ca10P046OH2 2F- - Ca10PO46F2 2OH OH ions are replaced hydroxy-apatite fluro-apatite Ref: Essentials of Pedodontics By Tilakraj, Pages 182-3 ; A Textbook of Public Health Dentistry By Marya, Page 348 