I believe your answer would be helium friend :)
Metal radiator. When hot water flows through the coils of the radiator , the metal heats up quickly by conduction and then radiates thermal energy into the surrounding air.
Answer: Metal radiator
I hope you understood
hit me up if you have any other questions :>
Answer:

Explanation:
C = Allowable concentration = 1.1 mg/L
= Flow rate of river = 
= Discharge from plant = 
= Background concentration = 0.69 mg/L
= Maximum concentration that of the pollutant
The concentration of the mixture will be

The maximum concentration that of the pollutant (in mg/L) that can be safely discharged from the wastewater treatment plant is
.