Assertion : Heavy water preferred over ordinary water as a moderator in reactors. Reason : Heavy water, used for slowing down the neutrons, has lesser absorption probability of neutrons than ordinary water.

Step-by-step solution

A bombarding particle gives up most energy when it makes an elastic collision with a particle of similar mass. For neutrons, hydrogen atoms would be most effective but unfortunately absorption occurs. However, deuterium (heavy water) and carbon (as graphite) are both suitable in which less absorption occurs. Therefore they are used as moderators.