The gas phase reaction $2A+B2C$ is to be carried out in a flow reactor for an equi$-$molar feed of A and $B.$A Levenspile plot for this reaction is shown below. a CSTR is used to reach conversion. a second reactor is needed to increase the conversion to $80\%.$ What type? and what volume?

a$.2400m^{3}PFR$

b$.1500m^3$ CSTR

c$.4000m^3$ CSTR

d$.900m^3$ PFR