What is the importance of heavy water?

(N/A) $i$. Heavy water $(D_2O)$ is extensively used as a moderator in nuclear reactors to slow down fast-moving neutrons.
$ii$. It is widely used as a tracer compound for studying the reaction mechanisms of various chemical and biochemical processes.
$iii$. Heavy water is used in the preparation of other deuterium compounds such as $CD_4$,$D_2SO_4$,etc.