The concentration of bod u in a river entering the first of two
lakes connected in series is equal to 20 g/m 3 . if the first-order
bod rate coefficient k is 0.35 d -1 and each lake behaves as a
complete-mix reactor, determine the bod u concentration in the
outlet of each lake. the steady-state river flow rate is 4,000 m 3
/d, and the lake volumes are 20,000 and 12,000 m 3 ,
respectively.

They are all decomposition reactions.

Electrical charges of ionic bonds

You would be correct, boiling water does disinfect contamination.    the property of disinfecting means to kill  germs, bacteria and other pathogens or contamination  that might cause an infection or cause a person to become sickly. a couple methods of disinfecting are using soap, other cleaning supplies or by simply "boiling" the contaminate.  the reason why boiling water disinfects certain attributes is because some cells cannot survive the heat and pressure.

The answer to this is the 3rd choice!