单项选择题

案例分析题

The search for a cheap alternative fuel to take the place of petroleum has led American scientists to study the potential of pond water.They are interested in a green scum known as algae that forms on the surface of stagnant pools.Helen Briggs reports from Washington：’Scientists have found a way of harnessing the natural ability of algae under certain conditions to make pure hydrogen gas from sunlight and water.’The researches based in California and Colorado describe the new discovery as the equivalent of striking oil.But there are still big obstacles to be overcome.The technique’s only been tried on a small scale in a laboratory and to be commercially viable，use of hydrogen and gas must be increased by a factor of ten.If they do manage to make the process work，perhaps in 20years’time，researchers say a small pond of algae could produce enough hydrogen gas to meet the weekly fuel needs for a dozen or so cars.

The search for a cheap alternative fuel has led American scientists to study the potential of（）