$($b$)$ Select the most favourable reactor system. OThe desired liquid$-$phase reaction

$A+BR+T,r_R=k_1{C}_A^2{C}_{B}0\mathrm{.}5$

is accompanied by the unwanted side reaction

$A+BS+U,r_S=k_2{C}_A{C}_B$

$($a$)$ From the standpoint of favourable product distribution $($Selectivity of $R$ over $S),$ how should the reactant concentrations be controlled?

Low $C_A,$ low $C_B$

High $C_A,$ low $C_B$

Low $C_A,$ high $C_B$

High $C_A,$ high $C_B$