R、L、C并联，接入一个频率可变的电源，开始时IR＝10A，IL＝15A，IC＝15

_ 小题1:__ People use money to buy food, furniture, books, bicycles and hundreds of other things they need or want. When they work, they usually get paid in money.

Most of the money today is made of metal or paper. __小题2:__ One of the first kinds of money was shells.

Shells were not the only things used as money. In China，cloth and knives were used. In the Philippine Islands，rice was used as money for a long time. Elephant tusks，monkey tails and salt were used as money in parts of Africa.

The first metal coins were made in China. They were round and had a square hole in the centre. __小题3:__

Different countries have used different metals and designs for their money. __小题4:__ Sweden and Russia used copper to make their money. Later some countries began to make coins of gold and silver.

But even gold and silver were inconvenient if you had to buy something expensive. Again the Chinese thought of a way to improve money. __小题5:__ The first paper money looked more like a note from one person to another than the paper money used today.   

Money has had an interesting history from the days of shell money until today.

A．The first coins in England were made of tin(锡)．

B．But people used to use all kinds of things as money.

C．No one knows for certain when people began to use money.

D．People strung(串连) them together and carried them from place to place.E. Money, as we know，is all made of paper.                               

F. They began to use paper money.

G. Today anyone will accept money in exchange for goods and services.

如果公司在下面哪些方面没有显著改变时可以用历史的β值估计权益成本( )。

A．经营杠杆

B．财务杠杆

C．收益的周期性

D．无风险利率

骨质疏松症的腰椎在X线片上可出现的影像有（）

A．扁平样变

B．双凹样变

C．椎体楔形改变

D．椎体骨皮质变薄

E．椎体骨小梁萎缩

汽轮发电机组负荷不变，循环水入口流量增加，真空（）。

A、不变；

B、降低；

C、升高；

D、可能降低也可能升高。

吸收塔采用带托盘的优点？