When $\mathrm{H}_2 \mathrm{~S}$ is passed through $\mathrm{Hg}_2 \mathrm{~S}$ we get

• A

  $\mathrm{HgS}$
• B

  $\mathrm{HgS}+\mathrm{Hg}_2 \mathrm{~S}$
• Correct answer
  C

  $\mathrm{Hg}_2 \mathrm{~S}$
• D

  $\mathrm{Hg}_2 \mathrm{~S}_2$

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