In the reaction between acidified K2Cr2O7 and iron (II) ions shown by the equation:

Cr2${\mathrm{O}}_{7}^{2-}$(aq) + 6Fe2+(aq) +14H+(aq)$\to$2Cr3+(aq) + 7H2O(l) + 6Fe3+(aq)

(a) the colour of the solution changes from green to blue

(b) the iron (II) ions are reduced

(c) thedichromate ions are reduced

(d) hydrogen ions are reduced