Correct Option: A

Heavy water, formally called deuterium oxide or 2H2O or D2O, is a form of water that contains the hydrogen isotope deuterium, rather than the common protium isotope. The colloquial term heavy water is often also used to refer a highly enriched water mixture that contains mostly deuterium oxide but also contains some ordinary water molecules as well: for instance heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction. In comparison, in ordinary water, there are only about 156 deuterium atoms per million hydrogen atoms). In its pure form, it has a density about 11% greater than water, but otherwise, is physically and chemically similar. Heavy water is 10.6% denser than ordinary water, a difference which is not immediately obvious.