In oder to achieve the same conversion under identical reaction conditions and feed flow rate for a non-autocatalytic reaction of positive order, the volume of an ideal $CSTR$ is​​​​​

1. always greater than that of an ideal $PFR$
2. always smaller than that of an ideal $PFR$
3. same as that of an ideal $PFR$
4. smaller than that of an ideal $PFR$ only for first order reaction