6.4. Reactant A (AR, CAD 26 mol/m³) passes in steady flow through four equal-size mixed flow reactors in series (Total = 2 min). When steady state is achieved the concentration of A is found to be 11, 5, 2, 1 mol/m³ in the four units. For this reaction, what must be plug so as to reduce CA from CAO 26 to CA = 1 mol/m³?