The liquid phase irreversible reaction $2$ A $->$ B has the reaction rate equation $-$rA $=$ k CA$2$ with k $=1$ L$/$gmol$.$men, carried out in a RATB with an inlet feed rate of $2$ L$/$min and an A concentration of $3$ gmol$/$L$.$ Calculate the reactor volume required to obtain $70\%$ A conversion if the reaction is carried out in :

a$.$ one RATB

b$.$ two RATBs in series with the same volume.