In Solvay process, $\mathrm{NH}_3$ is recovered when the solution containing $\mathrm{NH}_4 \mathrm{Cl}$ is treated with compound ' X '. What is ' X ' ?

• Correct answer
  A

  $\mathrm{Ca}(\mathrm{OH})_2$
• B

  $\mathrm{CaCl}_2$
• C

  NaOH
• D

  NaCl

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