Final answer:
In the use of coal to produce energy, cogeneration does not decrease the energy efficiency of the process, while extracting the coal from the ground, waste heat, the energy required to build the power plant, and the removal of the waste products all decrease the energy efficiency.
Step-by-step explanation:
In the use of coal to produce energy, extracting the coal from the ground, waste heat, the energy required to build the power plant, and the removal of the waste products all decrease the energy efficiency of the process. Cogeneration, on the other hand, does not decrease the energy efficiency. Cogeneration is the process of simultaneously producing electricity and useful heat from the same source, which helps increase the overall efficiency of the system.