Identify major product "X" formed in the following reaction :

The $$\mathrm{F}^{-}$$ ions make the enamel on teeth much harder by converting hydroxyapatite (the enamel on the surface of teeth) into much harder fluoroapatite having the formula.

Identify the incorrect statements regarding primary standard of titrimetric analysis.

(A) It should be purely available in dry form.

(B) It should not undergo chemical change in air.

(C) It should be hygroscopic and should react with another chemical instantaneously and stoichiometrically.

(D) It should be readily soluble in water.

(E) $$\mathrm{KMnO}_4$$ & $$\mathrm{NaOH}$$ can be used as primary standard.

Choose the correct answer from the options given below :

What is the structure of C?