In Nuclear Power Plants in the United States,near-Pure Water Functions

In nuclear power plants in the United States,near-pure water functions as

A) a moderator,heating up as it slows down neutrons,which triggers additional fission reactions.
B) a moderator,cooling down as it slows down neutrons that could cause fusion reactions.
C) an inhibitor,preventing neutrons from heating up and causing fusion reactions.
D) an inhibitor,preventing neutrons from cooling down and stopping the fission reactions.