Deficiency of `(F^(-))` in drinking water causes tooth decay. Its International Standard limit makes the enamal on tooth much harder by comverting hydroxyapatite into much harder

Assertion: The deficiency of fluoride in drinking water cause disease such as tooth decay etc. Reason: The F^(-) ions make the enamel on teeth much harder by converting hydroxyapatite, the enamal on the surface of the teeth, into much harder fluorapatite.

(A): Concentration of soluble F^(-) ions in drinking water about 1 ppm is essential. (R) Enamel on teeth is made much harder by converting hydroxyapatile 3Ca_(3)(PO_4)_2Ca(OH)_2 into 3Ca_(3)(PO_4)_2 CaF_2

The concentration of fluroide ions in drinking water upto 1 ppm make the enamel on teeth much harder by converting ul(x) into fluorapatite. What is ul(x) ?

Bone and tooth decay disease is caused by drinking water contaminated with