液面计有下列哪些情况的，应停止使用并更换（）A、阀件固死 B、出现假液位 C、液面计

当波从空气进入水中时，下列说法正确的是（　　）

A．频率不变，波速改变

B．频率改变，波速不变

C．波速大小改变，方向可能不变

D．波速大小及方向都一定改变

新生儿硬肿症的可能致病因素有________、________、________、________、________。

为什么说土壤是一类最好的“菌种资源库”？如何从土壤中分离、纯化菌种？

氧枪点不着火的原因是什么？

Scholars of human migration speak of two main factors that influence an individual’s decision to move from one place to another-push and pull factors. Push factors are associated with the place of origin. A push factor can be as simple and mild a matter as difficulty in finding a suitable job, or as traumatic as religious persecution, war, or severe famine Obviously, refugees who leave their homes with guns pointed at their heads or with hate-filled mobs at their heels are motivated almost entirely by push factors (although pull factors do influence their choice of destination).

Pull factors are those associated with the place of destination. Most of these are economic, such as better job opportunities or the availability of good land to farm. The latter was an important factor in attracting settlers to the United States during the nineteenth century. In general, pull factors add up to an apparently better chance for a good life and material well-being than is offered by the place of origin.

Besides push and pull factors, there are what the sociologists call " intervening obstacles " . Even if push and/or pull factors are very p, they still may be outweighed by intervening obstacles, such as the distance of the move, the trouble and cost of moving, the difficulty of entering the new country, and the problems likely to be encountered on arrival.

The decision to move is also influenced by " personal factors " of the potential migrant. The same push-pull factors and obstacles operate differently on different people, sometimes because they are at different stages of their lives, or just because of their varied abilities and personalities. The prospect of pulling up stakes and moving to a new and perhaps very strange environment may appear interesting and challenging to a young, footloose man and appallingly difficult to a slightly older man with a wife and young children. Similarly, the need to learn a new language and customs may intrigue one person and frighten another.

Regardless of why people move, migration of large numbers of people causes friction. The United States and other " receiving countries " (the term used for countries that welcome large numbers of migrants) have experienced adjustment problems with each new wave of immigrants. The newest arrivals are usually given the lowest-paying jobs and are resented by natives who may have to compete with them for those jobs. It has usually taken several decades for each group to gain acceptance in the mainstream of society in the receiving country.

Refugees migrate out of their country probably because of（）

A. their country’s unstable situation

B. bad economic conditions

C. guns pointing at them

D. their resentment towards government