Which of the following hypothetical discoveries, if made, would be most likely to reduce the technical difficulties in producing plastic from organic crops
A． a source of refining fossil fuels that significantly reduces their cost to plastic manufacturers B． a way of maintaining grain yields while producing high quantities of plastic that do not interfere with the functioning of plant chloroplasts C． a means of reducing the clean-up cost of environmental damage wrought by CO2 and SO2 emissions D． a means of yielding greater efficiency from coal-based fuels used in the organic plastic-production process E． the discovery of a renewable source of plant-sugar that would lessen reliance on fossil fuels in organic plastic-manufacture