Answer : Option B

Explanation :

The word “residual” means “remainder” or “that which is left”, and as the name suggests the chlorine residual is measure of the amount of chlorine remaining in the water after disinfection. The chlorine residual is usually tested in finished water which is ready to be released into the distribution system, although operators must also ensure that there is adequate residual at the extreme ends of the distribution system. Tests for chlorine residual are probably the most frequently performed tests at water treatment plants. There are three types of chlorine residual which must be considered in water treatment. Free chlorine residual - residual consisting of dissolved chlorine gas, Hypochlorous acid, and hypochlorite ions, Combined chlorine residual - residual consisting of other forms of chlorine such as chloramines which are capable of killing bacteria and oxidizing organic matter, Total chlorine residual - the sum of the free chlorine residual and the combined chlorine residual.