A. Experts such as Larry Burns, head of research at GM, reckon that only such a full hearted leap will allow the world to cope with the mass motorisation that will one day come to China or India. B. But once hydrogen is being produced from biomass or extracted from underground coal or made from water, using nuclear or renewable electricity, the way will be open for a huge reduction in carbon emissions from the whole system. C. In theory, once all the bugs have been sorted out, fuel cells should deliver better total fuel economy than any existing engines. D. That is twice as good as the internal combustion engine, but only five percentage points better than a diesel hybrid. E. Allowing for the resources needed to extract hydrogen from hydrocarbon, oil, coal or gas, the fuel cell has an efficiency of 30 %.