Which of the following statements is $ALWAYS\; CORRECT$ about an ideal chemostat?

1. Substrate concentration inside the chemostat is equal to that in the exit stream
2. Optimal dilution rate is lower than critical dilution rate
3. Biomass concentration increases with increase in dilution rate
4. Cell recirculation facilitates operation beyond critical dilution rate
1. $P$ and $Q$ only
2. $P, R$ and $S$ only
3. $P$ and $S$ only
4. $P, Q$ and $S$ only