$A+BB+B,-r_A=k{C}_A{C}_B$The following reaction is to be operated under the conditions of CA$0=90$ mol$/$m$3$ and CB$0=10$ mol$/$m$3.$

To minimize the overall volume of a reactor to obtain the given conversion rate, show which flow reactor or combination of reactors is optimal. It is not necessary to calculate the size of a reactor.

$1)20\%$ conversion

$2)90\%$ conversion