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:
-How many moles of B are present at 30 s?

A) 0.051
B) 0.073
C) 2.4 × 10^{- 3}
D) 1.7 × 10^{- 3}
E) 0.15