关于意大利米兰世博会，下列说法中正确的有哪些？ A: 米兰世博会历时3个月 B: 本次世博会为B类世博会 C: 米兰世博会总设计为Stefano Boeri D: 米兰世博会主题为“给养地球：生命的能源”

Passage OneQuestions 16 to 20 are based on the following passage.Immigration poses two main challenges for the rich world’s governments. One is how to manage the inflow (流入) of migrants; the other, how to integrate those who are already there.Whom, for example, to allow in? Already, many governments have realized that the market for top talent is global and competitive. Led by Canada and Australia, they are redesigning migration policies not just to admit, but actively to attract highly skilled immigrants. Germany, for instance, tentatively introduced a green card of its own two years ago for information-technology staff.Whereas the case for attracting the highly skilled is fast becoming conventional wisdom, a thornier issue is what to do about the unskilled. Because the difference in earnings is greatest in this sector, migration of the unskilled delivers the largest global economic gains. Moreover, wealthy, well-educated, ageing economies create lots of jobs for which their own workers have little appetite.So immigrants tend to cluster at the upper and lower ends of the skill spectrum. Immigrants either have university degrees or no high-school education. Mr. Smith’s survey makes the point: Among immigrants to America, the proportion with a postgraduate education, at 21%, is almost three times as high as in the native population; equally, the proportion with less than nine years of schooling, at 20%, is more than three times as high as that of the native-born.All this means that some immigrants do far better than others. The unskilled are the problem. Research by George Boras, a Harvard University professor whose parents were unskilled Cuban immigrants, has drawn attention to the fact that the unskilled account for a growing proportion of America’s foreign-born. Newcomers without high-school education not only drag down the wages of the poorest Americans; their children are also disproportionately likely to fail at school.These youngsters are there to stay. “The toothpaste is out of the tube,” says Mark Krikorian, executive director of the Centre for Immigration Studies. And their numbers will grow. Because the rich world’s women spurn motherhood, immigrants give birth to many of the rich world’s babies. Foreign mothers account for one birth in five in Switzerland and one in eight in Germany and Britain. If these children grow up underprivileged and undereducated, they will create a new underclass that may take many years to emerge from poverty.For Europe, immigration creates particular problems. Europe needs it even more than the United States because the continent is ageing faster than any other region. Immigration is not a permanent cure (immigrants grow old too), but it will buy time. And migration can “grease the wheels” of Europe’s sclerotic (硬化的) labor markets, argues Tito Boeri in a report published in July. However, thanks to the generosity of Europe’s welfare states, migration is also a sort of tax on immobile labor. And the more immobile Europeans are — the older, the less educated — the more xenophobic (恐惧外国人的) they are too.Q：It has become a generally accepted view that the rich governments should ________.