Temporary hardness of water can be removed by

Identify the statements which are not correct from the following:
a) In the structure of ice,each oxygen atom is surrounded by $4$ other hydrogen atoms via hydrogen bonding.
b) Temporary hardness of water is due to dissolved $Mg(HCO_3)_2$ and $Ca(HCO_3)_2$,not $NaHCO_3$.
c) In the reaction of acidified $KMnO_4$ and $H_2O_2$,$H_2O_2$ acts as a reducing agent.
d) $3 \ g \ L^{-1}$ $H_2O_2$ is not equal to $100$ volume $H_2O_2$ (since $100$ volume $H_2O_2$ is approximately $303.7 \ g \ L^{-1}$).

The velocity of neutrons in a nuclear reactor is slowed down by

When temporary hard water containing $Mg(HCO_3)_2$ is boiled,the precipitate formed is of:

Which among the following is the heaviest isotope of water?

Which of the following will cause softening of hard water?