Which of the following is used as a moderator in a nuclear reactor?

$D_2O$ is preferred to $H_2O$ as a moderator in nuclear reactors because:

Boiling of hard water is helpful in removing the temporary hardness by converting calcium hydrogen carbonate and magnesium hydrogen carbonate to ....

Which pairs of ions are responsible for hardness of water?

Given below are two statements:
Statement-$I$: Permutit process is more efficient compared to the synthetic resin method for the softening of water.
Statement-$II$: Synthetic resin method results in the formation of soluble sodium salts.
In the light of the above statements,choose the most appropriate answer from the options given below:

The photolysis of water gives the substance