Concept:Fast-moving neutrons are slowed down by a moderator through collisions.
Explanation:In a nuclear reactor, fission releases neutrons at very high speed.
These fast neutrons must be slowed to maintain a steady chain reaction.
This slowing process is called moderation.
A moderator is a material that absorbs kinetic energy from neutrons.
Graphite is commonly used as a moderator, often in the form of graphite rods.
Graphite has light nuclei, which are effective in reducing neutron speed.
Concrete shields protect against radiation and do not moderate neutrons.
Iron rods are not used as moderators in reactors.
Boron rods act as control rods that absorb neutrons instead of slowing them.
Therefore, the correct method is using graphite rods.
Answer:A. graphite rods.