听力原文： Are electric cars the way of the future?Automobile manufactures are under pressure to develop cars that do not pollute.One powerful motive is a California law requiring that by the year 2020 ten percent of new car sells in the state be so called zero-emission vehicles.These cars must put no pollutants whatsoever into the at-mosphere. California is a huge market for the automobile companies,so they are working hard to meet these standards. So far the electric car seems to be the best alternative.So the big advantage of electric cars is that they don't pollute.However they will be in competition with gas-powered cars and that's where the weaknesses come out.The big problem is that the batteries that power electric cars weigh a lot relative to the amount of power they deliver.For instance,in one prototype electric car,the batteries weighed 400 kilograms.And they provide enough energy to go 250 kilometers before recharging,which takes eight hours.Compare that to a moderately fuel-efficient conventional car,it can go 600 to 700 kilometers on a tank of gas and refilling takes just minutes.If there are other drains on an electric car's batteries besides the motor,headlights,air-conditioning or a heater,its already limited range will be significantly reduced.So automobile engineers are trying to make more powerful batteries that would increase the cars'range and make them more attractive to buyers. Why does the electric car seem to be the best alternative so far?