Final answer:
A gas water heater is more efficient overall than an electric one when electricity comes from a coal-fired power plant mainly due to the electric heater's lower direct efficiency and the coal plant's low energy conversion rate. Transportation of natural gas isn't a considerable factor in the efficiency of the water heater itself, so the correct options are i and ii only.
Step-by-step explanation:
The question asks why a gas water heater is more energy efficient overall than an electric water heater when the electricity comes from a coal-fired power plant. The factors to consider are the direct efficiency of the water heaters, the efficiency of the coal-fired power plant in converting thermal energy to electricity, and the transportation of natural gas.
Firstly, while an electric water heater may be 100% efficient in transferring delivered energy into the water, this doesn't take into account the energy losses in producing and transmitting that electricity. A coal-fired power plant typically converts only about 40% of the energy from coal into electricity. The rest of the energy is lost as heat to the environment.
Secondly, even though natural gas must be transported, this doesn't significantly affect the comparison between gas and electric water heaters, as both fuel sources require some form of transportation, and it's not a primary factor in energy efficiency of the device itself.
Therefore, the correct answer is c. i and ii only, as the electric water heater has lower overall energy efficiency due to the inefficiency of the coal-fired power plant.