Effective moderators are those materials which contain

A Light weight atoms
B Heavy weight atoms
C Critical atoms
D Zero weight atoms

Correct Answer: Light weight atoms

Moderators are made from materials with light nuclei which do not absorb the neutrons but rather slow them down by a series of collisions.
Carbon in the form of graphite is a material used for moderators as is heavy water which is Deuterium an isotope of Hydrogen with an atomic mass of 2 bonded to Oxygen.
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