The enamel present on teeth becomes much harder due to the conversion of $\left[3 \mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2, \mathrm{X}\right]$ into $\left[3 \mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2, \mathrm{Y}\right]$. What are X and Y ?

• Correct answer
  A

  $\mathrm{X}=\mathrm{Ca}(\mathrm{OH})_2, \mathrm{Y}=\mathrm{CaF}_2$
• B

  $\mathrm{X}=\mathrm{Ca}(\mathrm{OH})_2, \mathrm{Y}=\mathrm{CaCl}_2$
• C

  $\mathrm{X}=\mathrm{Ca}(\mathrm{OH})_2, \mathrm{Y}=\mathrm{NaCl}$
• D

  $\mathrm{X}=\mathrm{CaO}, \mathrm{Y}=\mathrm{CaCl}_2$

Hence, the correct answer is option: 'A'.