A、When it is used in rural areas. B、When it is environment-friendly. C、When it operates at near capacity. D、When it operates at

Every year, people in America use energy equal to over 30 million barrels of oil each day. That’s energy in all forms: oil, gas, coal, nuclear, solar and even wind power. Viewed from a scientist’s standpoint, all of the energy contained in fuel, either now or in the future, becomes heat. Some of the heat is used directly or produces useful work. The rest is lost or rejected, radiated into the atmosphere from the engines, motors, boilers and all the other energy-consuming machinery that makes America’s wheels go around. If we could figure out how to improve the efficiency with which we use energy, we could do a lot more work with the energy that is available. With money and careful planning, we can boost efficiency up to a point. Yet improvement won’t come easily. It won’t come overnight and there are limits beyond which not even science can take us. According to the Centre for Strategic and International Studies, about three quarters of the energy we use to move things, including ourselves, accomplishes no useful work. In terms of efficiency and cleanness, buses, trains and other forms of public or mass transportation are superior to the private automobile, but only if they operate at near capacity. Otherwise, their overall efficiency is poor. For some people, mass transit may answer all transportation needs. For others, a combination of mass transit and private transportation may be desirable. Better design and wiser use of both mass transit systems and private vehicles will play an important part in helping America get more out of energy used for transportation.
    12.What does the speaker say about energy used in the United States?
    13.What do scientists say about energy contained in fuel?
    14.When does mass transit prove superior to private vehicles?
    15.What seems to be the speaker’s biggest concern?