The following reaction rate curve is shown for a reaction $A{\rightarrow}P$. Here, $(-r_{A})$ and $X_{A}$ represent reaction rate and conversion, respectively. The feed is pure $A$ and $90\%$ conversion is desired.

Which amongst the following reactor configuration gives the lowest total volume of the reactor$(s)$?

1. $CSTR$ followed by $PFR$
2. Two $CSTRs$ in series
3. $PFR$ followed by $CSTR$
4. A single $PFR$