属于含氟牙膏中成分的是（） A.硝酸钾 B.焦磷酸盐 C.西比氯胺 D.五倍子 E.

如图甲所示，一个正方形导线圈的边长a=0.2m，共有N=100匝，其总电阻r=4Ω，线圈与阻值R=16Ω的外电阻连成闭合回路．线圈所在区域存在着匀强磁场，磁场方向垂直线圈所在平面向外，磁感应强度的大小随时间作周期性变化，如图乙所示，试求：

（1）在开始的0.01s时间内通过电阻R的电流大小；

（2）在 t=3s内，电阻R中所产生的热量．

There seems never to have been a civilization without toys, but when and how they developed is unknown. They probably came about just to give children something to do .

In the ancient world , as is today ,most boys played with some kinds of toys and most girls with another .In societies where social roles are rigidly determined, boys pattern their play after the activities of their fathers and girls are being prepared, even in play , to step into the roles and responsibilities of the adult world .

What is remarkable about the history of toys is not so much how they changed over the centuries but how much they have remained the same . The changes have been mostly in terms of craftsmanship ,mechanics, and technology . It is the universality of toys with regard to their development in all parts of the world and their persistence to the present that is amazing .In Egypt ,America ,China ,Japan and among the Arctic (北极的)people, generally the same kinds of toys appeared. Variations depended on local customs and way of life because toys imitate their surrounding. Nearly every civilization had dolls, little weapons, toy soldiers, tiny animals and vehicles.

Because toys can be generally regarded as a kind of art form, they have not been subject to technological leaps that characterize inventions for adult use .The progress from the wheel to the cart to the automobile is a direct line of ways up. The progress from a rattle (拔浪鼓) used by a baby in 3000BCto one used by an infant today , however , is not characterized by inventiveness. Each rattle is the product of the artistic tastes of the times and subject to the limitations of available materials.

小题1:The reason why the toys most boys play with are different from those that girls play with is that       .

A．their social roles are rigidly determined

B．they like challenging activities

C．most boys would like to follow their fathers’ professions

D．boys like to play with their fathers while girls with their mothers.小题2:One aspect of “the universality of toys ”lies in the fact that          .

A．the basic characteristics of toys are the same all over the world

B．technological advances have greatly improved the durability of toys

C．the exploration of the universe has led to the creation of new kinds of toys

D．the improvement of craftsmanship in making toys depends on the efforts of universities小题3:Which of the following is the author’s view on the historical development of toys?

A．Toys are playing an increasingly important role in shaping a child’s character.

B．The toy industry has witnessed great leaps in technology in recent years.

C．The craftsmanship in toy-making has remained essentially unchanged.

D．Toys have remained basically the same all through the centuries.小题4:Regarded as a kind of art form, toys          .

A．reflect the pace of social progress

B．are not characterized by technological progress

C．follow a direct line of ascent

D．also appeal greatly to adults

分馏塔底液面下降，塔底温度和蒸发段温度上升。

燃料油在到达燃烧器之前经过几级加热？

允许速度120km/h以下的区段，单根更换木枕应每隔六根轨枕抽换1根。