Based on the reactions and data below, which reaction is least spontaneous?
A. `2H^(2+) + 2e^(-) rightarrow Hg^(2+_(2)) + I_(2)`
B. `H_(2) + 2OH^(-) + I_(2) rightarrow 2H_(2)O + 2I^(-)`
C. `2H_(2)O + Hg_(2)^(2+) rightarrow H_(2) + 2OH^(-) + 2Hg^(2+)`
D. `H_(2) + 2OH^(-) + 2Hg^(2+) rightarrow 2H_(2)O + Hg_(2)^(2+)`