[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 reside