（）开始，早期公司形态得到了普遍发展。 A.11世纪 B.13世纪 C.15世纪 D

生态系统是由______和______两部分组成．

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 (53) of fifteen hours a week on a daytime 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 European 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.

53().

A.natural 

B.financial 

C.human 

D.economical

已知X、Y、Z都是短周期的元素，它们的原子序数依次递增。X原子的电子层数与它的核外电子总数相同，而Z原子的最外层电子数是次外层的三倍，Y和Z可以形成两种以上气态化合物，则

（1）X是________ ，Y是____________，Z是____________。

（2）由Y和Z组成，且Y和Z的质量 比为7：20的化合物的化学式（分子式）是____________。

（3）由X、Y、Z中的两种元素组成，且与X、Z分子具有相同电子数的两种离子是___________ 和 ______________。

（4）X、Y、Z可以形成一种盐，此盐中X、Y、Z元素的原子的个数比为4：2：3，该盐的化学式（分子式）是_____________。

为什么液压操作机构的断路器在压力过低时不允许操作？

下列哪些疾病要与肠结核相鉴别（）

A.急性闌尾炎

B.右侧结肠癌

C.阿米巴病

D.血吸虫病性肉芽肿

E.Crohn病