根据以下材料，回答

There is little debate about the firstdevelopment of the energy system of the 22nd century--that most of the energyproduced on a large scale will be in the form of electric power. Thus, most presentconcepts for major long-term energy sources (solar, geothermal, fusion) are basedupon electric power production at central plants.

The second major development is thepotential use of hydrogen as both a basic fuel to replace natural gas and aspart of a general system to convert, store and transmit electric energy overlong district by fuel cells, or it could be used directly as a fuel forcommercial, industrial or transportation purposes.

Although hydrogen seems likely to play amajor role in the future as a form of energy transport and storage, other processesmay also be useful. Here we note only two possibilities.

Flywheels: It appears to be workable toconstruct flywheels of almost any needed size that would be efficientconverters (93-95 percent) in first storing and then releasing electric power.In addition, the kinetic (动力的) energy ofrotation is easily converted to alternating electric power. But the engineeringis at an early stage of development, and a decade or more may pass before this potentialcan be reliably estimated.

Batteries and fuel cells: some of the manybatteries being developed today look quite promising. Within a decade or two, abattery-operated electric auto might even become competitive with today′ s vehicles,at least for driving ranges up to 200 miles. An electric automobile would bepollution-free,and would probably be more energy-efficient than theinternal-combustion (内燃) engine.

A fuel cell (which may be considered a hydridebattery) with about 40 percent conversion efficiency is expected to enter powerproduction on a large scale within about five years. Unlike steam-poweredgenerators, fuel cells can be made large or small and their efficiency inoperation is essentially independent of the loading. Thus t

• A.solar energy
• B.geothermal energy
• C.hydrogen
• D.electricity