$($a$)$ Give detailed note clearly differentiating the following terms as applied to water treatment: i$)$ Coagulation and flocculation ii$)$ Fluoridation and de$-$ fluoridation iii$)$ Double Filtration and In$-$depth Filtration iv$)$ Water Softening and Stabilisation.

$($b$)$ How do thermal inversions relate to air pollution $$events$?$