题目：
Visitors to this country are normally admitted for six months, but foreign students can usually stay for one year. They must (52) an educational institution, and they are required to study for a minimum of fifteen hours a week on a (53) course. Prospective students have to show that they can afford their studies, and that they have sufficient (54) resources to support themselves (55) in this country.
In order to work here the foreigner needs a work permit, (56) must be applied for by his prospective employer. The problem here is that the Department of Employment has the right to grant or (57) these permits, and there is little that can be done (58) it; it would be extremely unwise for a foreign visitor to work (59) a permit, since anyone doing so is liable to immediate deportation (驱逐出境). There are some people from the Common Market countries, who are often given (60) residence permits of up to five years. Some other people, such as doctors, foreign journalists, authors and others can work without permits, and foreign students are normally allowed to (61) part-time jobs while they are studying here.
55().
A.after
B.but
C.while
D.before
答案：
参考答案：C
解析：
此处应该填while，该句是一个省略句，省略了they were。当从句的主语与主句的主语一致时，而从句又是主系表结构，就可以省略从句中的主语和系动词。
