If $90\%$ of reactant B is converted into product by a second$-$order reaction in a single mixed flow reactor. For the same conversion by how much the treatment rate can be increased if we plan to place a second order reactor like the one being used in series.

$27\mathrm{.}5$

$7\mathrm{.}06$

$6\mathrm{.}6$

$26\mathrm{.}5$