In the cell represent as $\mathrm{Ni}_{(\mathrm{s})}\left|\mathrm{Ni}_{(\mathrm{MM})}^{2^{\oplus}}\right|\left|\mathrm{Ag}_{(\mathrm{IM})}^{\oplus}\right| \mathrm{Ag}_{(\mathrm{s})}$, the reducing agent is