Hydrogen peroxide acts both as an oxidising and as a reducing agent depending upon the nature of the reacting species. In which of the following cases `H_(2)O_(2)` acts as a reducing agent in acid medium ?
A. `MnO_(4)^(-)`
B. `SO_(3)^(2-)`
C. `KI`
D. `Cr_(2)O_(7)^(2-)`