Nuclear Power Engineering
Water is a better coolant than a gas (like CO₂, He, N₂ etc.), because it

Is a better neutron moderator as well
Requires comparatively smaller pumps and heat exchanger for a given heat transfer rate
All of these
Has a better heat transfer characteristics, and it can be pressurised to attain a high temperature

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Nuclear Power Engineering
Molten sodium (as a coolant in fast breeder reactor)

Is not at all corrosive, even at a higher temperature
Is highly radioactive at elevated temperatures and can cause explosion, when it comes in contact with air or water
None of these
Can't attain high temperature at normal pressure

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