A Flask Is Charged with 0

A flask is charged with 0.124 mol of A and allowed to react to form B according to the reaction A(g) -B(g) . The following data are obtained for [A] as the reaction proceeds:
-The average rate of disappearance of A between 20 s and 40 s is mol/s.

A) 1.4 × 10^{- 3}
B) 590
C) 8.5 × 10^{- 4}
D) 7.1 × 10^{- 3}
E) 1.7 × 10^{- 3}