$3.$ Bleaching powder is produced by the reaction of $\_($a$)\_$ with chlorine gas, resulting in the formation of $\_($b$)\_$ and $\_($c$)\_.$

$-($a: Calcium hydroxide, Sodium hydroxide, Magnesium hydroxide$)$

$-($b: Calcium hypochlorite, Sodium chloride, Calcium carbonate$)$

$-($c: Calcium chloride, Water, Calcium carbonate$)$

$4.$ The mercury cell process for caustic soda production involves the formation of $\_($a$)\_$ at the cathode, which reacts with water to produce $\_($b$)\_$ and $\_($c$)\_.$

$-($a: Sodium amalgam, Sodium, Sodium hydroxide$)$

$-($b: Sodium hydroxide, Sodium chloride, Chlorine gas$)$

$-($c: Hydrogen gas, Water, Oxygen gas$)$