Which of the following half-reaction belongs to a secondary cell ?

• A

  Pb + ${\mathrm{SO}}_4^{-2}$$\to$PbSO₄ + 2e^-

• B

  MnO₂ + ${\mathrm{NH}}_4^{+}$ + e^- $\to$ MnO(OH)

• C

  HgO + H₂O + 2e^-$\to$Hg + 2 OH^-

• D

  Zn + 2 OH^-$\to$ZnO + H₂O + 2e^-