Sulphides of which of the following metal ions do not form black precipitate?

• A

  $\mathrm{Hg}^{2+}$ and $\mathrm{Co}^{2+}$
• Correct answer
  B

  $\mathrm{Cd}^{2+}$ and $\mathrm{Sb}^{3+}$
• C

  $\mathrm{Ni}^{2+}$ and $\mathrm{Ag}^{+}$
• D

  $\mathrm{Co}^{2+}$ and $\mathrm{Cu}^{2+}$

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