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During the day, Leipzig's airport is quiet. It is at night that the airfield comes to life. Next to the runway a yellow warehouse serves as the global sorting hub for DHL, a delivery firm owned by Deutsche Post of Germany. A huge extension, which opened in October, means it can sort 150,000 parcels each hour, says Ken Allen, DHL's CEO. With falling trade barrier, cross-border e-commerce has become a key term in the modern economy. The rise of cross-border e-commerce has meant booming business for express-delivery firms. On January 31st UPS revealed record revenues for the fourth quarter of 2016; FedEx and DHL are expected to report similarly buoyant results next month. Since 2008 half of the increase in express-delivery volumes has come from shoppers buying items online from another country. Falling trade barriers have greatly helped them. When DHL and FedEx were getting going in the 1970s, there was little demand for international express deliveries. Packages often got stuck in customs for weeks and were heavily taxed. The expansion of free-trade areas, lower tariffs and the Internet brought years of growth. But after Mr. Trump's threats to raise tariffs on goods from China and Mexico, together with the indication last month from Theresa May, Britain's prime minister, that the country will leave the EU's customs union, there are widespread fears that the favourable tailwinds enjoyed by the industry for decades are gone. The express-delivery industry faces a new challenge; the return of trade barriers due to the protectionist bent of Donald Trump and because of Brexit. The return of borders poses a challenge to the soaring parcel-delivery business. " It's all a real nightmare," groans David Jinks of ParcelHero, a British parcel broker which works with DHL, FedEx and UPS. Start with Brexit. Post-Brexit costs will probably come from long wrangles over which of 19,000 customs codes should be applied to a consignment. As an example of what could happen, Halloween costumes from China often get stuck at Britain's border while customs officials work out whether they are toys or children's clothes, which attract different duties. Such complexity would force delivery firms to put up their prices to customers, Mr. Jinks says. Sending an item from Britain to Switzerland (outside the EU) costs 150% more than it does to Italy (inside the EU). The most severe impact on business would come from higher tariffs, which would hurt demand for cross-border imports and deliveries in favour of local goods. This is where Mr. Trump's threats come into focus.
单选题21.The example of Leipzig's airport shows that______.