Consider the Reaction in Aqueous Solution

Consider the reaction in aqueous solution:
3 I ^- + S₂O₈^{2 -} →I₃ ^- + 2 SO₄^{2 -} . Under a given set of conditions, the concentration of I₃ ^- changes from 0 to 7.6×10 ^{- 4} M in 20 seconds. Calculate D [I ^- ]/ D t (in mol/L × s) for this time interval.

A) 3.8×10 ^{- 5}
B) - 2.5×10 ^{- 5}
C) - 3.8×10 ^{- 5}
D) - 1.1×10 ^{- 4}
E) 7.6×10 ^{- 5}