Calculate the oxidation number of s in S2O8^2-

Given problem:

    S₂O₈²⁻

Find the oxidation number of S;

Oxidation number presents the extent of oxidation of each atom of elements a molecular formular or formula unit or an ionic radical.

  For radicals:

          "the algebraic sum of all the oxidation numbers of all atoms in an ion containing more than one kind of atom is equal to the charge on the ion  "

 S₂O₈²⁻;   oxidation number of O is usually -2

             2(S) + 8(-2) = -2

               2S - 16  = -2

               2S = -2 + 16

                2S  = 14

                   S  = +7

The oxidation state of S in the radical is +7

the answer is either b or c