The correct option is: 4. Water only.
Insulation is a material that serves as a barrier between the interior of your home and the considerably variable outside temperature to decrease heat absorption or loss. Heat cannot be added or removed from the vessel since it is insulated; consequently, there is no heat exchange. If hot water and cold water are combined, the final water temperature in the equilibrium vessel will be moderate. Hot water generates the same amount of heat that cold water looses.
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The complete question is:
In an insulated vessel, a quantity of hot water at temperature T1 is mixed with a different quantity of cold water at temperature T2. After equilibrium is established, the vessel contains __________.