【单选题】

Many people consider the wider use of biofuels a promising way of reducing the amount of surplus carbon dioxideCO₂) being pumped into the air by the world’s mechanized transport. The theory is that plants such as sugar cane, maize (corn, toAmericans), oilseed rape and wheat take upCO₂ during their growth, so burning fuels made from them should have no net effect on the amount of that gas in the atmosphere.

Theory, though, does not always translate into practice, and just as governments have committed themselves to the greater use of biofuels, questions are being raised about how green this form of energy really is. The latest comes from the InternationalCouncil for Science (ICSU) based in Paris.
The ICSU report concludes that, so far, the production of biofuels has aggravated rather than ameliorated global warming. In particular, it supports some controversial findings published in 2007 by PaulCrutzen of the Max Planck Institute forChemistry in Germany.Dr.Crutzen concluded that most analyses had underestimated the importance to global warming of a gas called nitrous oxide (N₂O). The amount of this gas released by farming biofuel crops such as maize and rape probably negates by itself any advantage offered by reduced emissions ofCO₂.
Although N₂O is not common in theEarth’s atmosphere, it is a more potent greenhouse gas thanCO₂ and it hangs around longer. The result is that, over the course of a century, its ability to warm the planet is almost 300 times that of an equivalent mass ofCO₂.
N₂O is made by bacteria that live in soil and water and, these days, their raw material is often the nitrogen-rich fertiliser that modern farming requires. Since the 1960s the amount of fertiliser used by farmers has increased sixfold, and not all of that extra nitrogen ends up in their crops. Maize, in particular, is described by experts in the field as a “nitrogen-leaky” plant because it has shallow roots and takes up nitrogen for only a few months of the year. This would make maize (which is one of the main sources of biofuel) a particularly bad contributor to global N₂O emissions.
But it is not just biofuels that are to blame. The ICSU report suggests N₂O emissions in general are probably more important than had been realiseD、Previous studies, including those by the International Panel onClimateChange (IPCC、, a United Nations-appointed body of experts, may have miscalculated their significance — and according toAdrian Williams ofCranfield University, inBritain, even the IPCC’s approach suggests that the global-warming potential of most ofBritain’s annual crops is dominated by N₂O emissions.
Biofuels are appreciated by governments becauseA．burning biofuels does not addCO₂ to the atmosphere.
B．biofuels can slow down the pace of global warming.
C．biofuels is a promising and profitable form of green energy.
D．burning biofuels discharges less N₂O than burning oils.