Water softening by Clark's process uses

In solid ice, an oxygen atom is surrounded by:

Pure de-mineralised water can be obtained by

Some statements about heavy water are given below:
$(i)$ Heavy water is used as a moderator in nuclear reactors.
$(ii)$ Heavy water is more associated than ordinary water.
$(iii)$ Heavy water is more effective solvent than ordinary water.
Which of the above statements are correct?

Which of the following statements is/are correct?

$D_2O$ is used in