Right now, Prince Charles is probably wish

问题：

Right now, Prince Charles is probably wishing he had hit the slopes after all. Britain’s Prince of Wales decided last year to begin reducing his carbon footprint--the amount of carbon dioxide created by his activities--by cutting down on his flights abroad, including an annual skiing vacation in Switzerland. Though we should all be in the position to make such sacrifices, Charles didn’t win plaudits for his holiday martyrdom. Instead British green groups, seconded by Environment Secretary David Miliband, spanked the Prince for deciding to fly to the U. S. on Jan. 27 to pick up a prestigious environmental award, arguing that the carbon emissions created by his travel canceled out his green cred.
It’s too easy to mock His Royal Highness; in England it’s practically the national sport. But his critics may be onto something. Jets are uniquely polluting, and the carbon they emit at high altitudes appears to have a greater warming effect than the same amount of carbon released on the ground by cars or factories. On an individual level, a single long-haul flight can emit more carbon per passenger than months of SUV driving. Though air travel is responsible for only 1.6% of total greenhouse gas emissions, in many countries it’s the fastest-growing single source--and with annual airline passengers worldwide predicted to double to 9 billion by 2025, that growth is unlikely to abate. The Intergovernmental Panel on Climate Change (IPCC) put it bluntly last year: "The growth in aviation and the need to address climate change cannot be reconciled. "
One of the biggest problems, as the IPCC points out, is that the carbon emitted by air travel currently has "no tech no fix. " As messy a source of pollution as electricity generation and ground transportation are, technologies do exist that could drastically cut carbon from power plants and cars. Not so for planes, the same aircraft models will almost certainly be flying on the same kerosene fuel for decades.
Admittedly, the airline industry has improved efficiency over the past 40 years, with technological upgrades more than doubling efficiency. There are tweaks in aircraft operations that could nip carbon emissions even further. Virgin Atlantic airlines tycoon Richard Branson, who pledged $ 3 billion in the fight against climate change, advocates having planes towed on the ground rather than taxiing, which he has said could cut a yet unspecified portion of fuel on long flights. Emissions trading for the air industry could help as well, with airlines given carbon caps and then being required to purchase credits from other industries if they exceed their limits. But there’s nothing on the horizon for aircraft with the carbon- cutting potential of hydrogen engines or solar energy. "It’s not like having leaky home windows you can fix with double glazing," says Leo Murray, a spokesman for the green group Plane Stupid, which led the criticism of Prince Charles. Nor is there any replacement for long-haul air travel itself. I can take a train from Boston to Washington, but until we can figure out how to travel via fireplace, Harry Potter-style, the only way I’m getting from Tokyo to New York City is in aircraft that may emit more than 5,200 Ibs. (about 2,400 kg) of carbon per passenger, round-trip, according to one estimate. On an individual level, you can try to make your flight carbon neutral by donating to, say, a forestry project that will soak up the greenhouse gases you have created. An increasing number of airlines and travel agents do offer such options. The London-based CarbonNeutral Company reports that requests for carbon offsetting from individual travelers have jumped over the past six months. But the still tiny number of neutralized flights can hardly compensate for the rapid increases in global air travel.
So is grounding ourselves the only answer That seems to be the conclusion of environmentalists in Britain, who also went after Prime Minister Tony Blair for a recent holiday trip to Miami. Though Blair belatedly promised to begin offsetting his leisure travel, he insisted that telling people to fly less was simply impractical--and he’s probably right. Some environmentalists suggest that we could learn to live more locally, but good luck keeping them in Brighton after they’ve seen Beijing--and vice versa. Our best bet for now may be to limit any business and leisure flights that we can and offset the rest. So when you’re pondering that luxury Swiss vacation, ask yourself: What would Prince Charles do

The word "tweaks" in the sentence "There are tweaks in aircraft operations that could nip carbon emissions even further. "(para. 4) can be paraphrased as ______.

A．theoretical possibilities

B．great inventions

C．minor improvements

D．technological upgrades