Which of the following can be added to remove the permanent hardness of water?

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

Which of the following compounds is used for the softening of hard water by Clark's method?

Describe the structure of the common form of ice.

The decreasing order of hydrogen bonding in the following forms of water is correctly represented by:
$A.$ Liquid water
$B.$ Ice
$C.$ Impure water

Which process is used for the removal of hardness of water?