Match the items in Group – I with those in Group – II and choose the right combination.
$$\begin{array}{llll} {} & \underline{\text{Group – I}} & {} & \underline{\text{Group – II}} \\ P. & \text{Activated sludge process} & 1. & \text{Nitrifiers and denitrifiers} \\ Q. & \text{Rising of sludge} & 2. & \text{Autotrophic bacteria} \\ R. & \text{Conventional nitrification} & 3. & \text{Heterotrophic bacteria} \\ S. & \text{Biological nitrogen removal} & 4. & \text{Denitrifiers} \end{array}$$
- $P – 3; \: Q – 4; \: R – 2; \: S – 1$
- $P – 2; \: Q – 3; \: R – 4; \: S – 1$
- $P – 3; \: Q – 2; \: R – 4; \: S – 1$
- $P – 1; \: Q – 4; \: R – 2; \: S – 3$