Heavy water and graphite are two examples of materials that are can be used in a nuclear reactor to slow down neutrons. These materials are called:
A. fuels.
B. shields.
C. coolants.
D. moderators.
E. inhibitors.
Do not be tricked into picking water as a coolant. The question did not ask for a coolant; it asked for "traffic cops" that slow down or moderate or keep "the lid on" a reaction.