The cold reservoir is used to discard waste heat. Waste heat is heat that cannot be converted into mechanical energy.
Why must every heat engine have a cold reservoir?
a) Because it is impossible for even a perfect engine to convert heat entirely into mechanical work.
b) Because the cold reservoir keeps the engine from overheating.
c) Because the cold reservoir keeps the engine from overcooling.
d) Because the cold reservoir increases the power of the engine.