填空题

Questions 47 to 51 are based on the following passage.
According to the latest OECD Migration Outlook, America received 1,107,000 permanent immigrants in 2008. About 73% of them came for family re-unification, which often means they are unskilled. About 15% came as refugees, and only 7% were labour migrants, meaning they came for work. There were also 340,700 temporary migrants who came on student visas. So much family and refugee migration makes sense for humanitarian reasons, but does it make sense economically The American economy would benefit from more skilled workers, so why do they make up such a small fraction of migrant flows
The low fraction of labour migrants in America is due to the few work visas available. Most labour migrants must have an American employer sponsor them. Most skilled workers initially come as temporary migrants under an H1-B visa. After a few years, if your employer sponsors you, this can be converted into permanent residency.
It may seem counter-productive to want more labour migrants when unemployment is high, but immigration can actually be a source of job creation. Research from the Kauffman Foundation has found that more than half of all Silicon Valley start-ups had at least one foreign-born founder. Jennifer Hunt, an economist, has found that immigrants, who come as either students or on a H1-B, are more likely than natives to file a patent and make a profit with their innovation. But you need an employer sponsor for an H1-B. So when you migrate on this visa it’s hard, at least initially, to be self-employed. While there’s evidence that this is an exceptionally entrepreneurial population, America limits its numbers and designs visas to discourage entrepreneurship(创业).
The question for immigration policy reform should be how America can attract immigrants who will contribute most to economic growth. There are good reasons for the absolute number of family and humanitarian migrants. It is important to keep in mind that low-skill migrants also make a significant contribution to the American economy. But it seems odd that America makes it so hard for skilled migrants to come for work. Expanding the number of H1-Bs would be a good start. But it should also consider policies, already in place in Britain and Australia, which allow skilled migrants and students to come and work in America based on their skills and achievements.
Kauffman Foundation’s research is cited to prove that immigration is a source of ______.