The reducing agent that can convert $1\mathrm{.}0mo\frac{l}{L}S{n}^{4+}(aq)$ ions to $S{n}^{2+}(aq)$ but not $1\mathrm{.}0mo\frac{l}{L}S{n}^{2+}(aq)$ to $Sn(s)$ is

A$.Cu(s)$

B$.Pb(s)$

C$.Ni(s)$

D$.Cr(s)$