When two isothermal PERs in series are used in place of a single isothermal PER at the same temperature, the total reactor volume of the series (i.e., the sum of the two reactors) required for a given conversion will be (a). the same as that of a single PFR (b). less than that of a single PER (c). more than that of a single PER different from that of a single (d). PER, but depends on the reactor temperature