分段结算是针对当年开工但当年不能竣工的单项工程或单位工程，按照( )，划分不同阶段

如图所示，质量m=8.0kg的物体放在光滑水平地面上．对于物体施加一个F=44N的水平拉力，使物体由静止开始做匀加速直线运动．求：

（1）物体加速度a的大小；

（2）物体在t=3.0s时速度v的大小．

如图所示，宽度为d的竖直狭长区域内(边界为L₁、L₂)，存在垂直纸面向里的匀强磁场和竖直方向上的周期性变化的电场(如图所示)，电场强度的大小为E₀，E＞0表示电场方向竖直向上．t＝0时，一带正电、质量为m的微粒从左边界上的N₁点以水平速度v射入该区域，沿直线运动到Q点后，做二次完整的圆周运动（其轨迹恰好不穿出边界L₁），以后可能重复该运动形式，最后从边界L₂穿出．重力加速度为g，上述d、E₀、m、v、g为已知量．

（1）求该微粒通过Q点瞬间的加速度；

（2）求磁感应强度B的大小和电场变化的周期T；

（3）若微粒做圆周运动的轨道半径为R，而d=4.5R，使微粒仍能按上述运动过程通过相应宽度的区域，求微粒所用的时间．

Everywhere man is altering the balance of nature．He is facilitating the spread of plants and animals into new regions, sometimes deliberately, sometimes unconsciously．He is covering huge areas with new kinds of plants, or with houses, factories, slag-heaps and other products of his civilization．He exterminates some species on a large scale, but favours the multiplication of others．In brief, he has done more in five thousand years to alter the biological aspect of the planet than has nature in five million．

Many of these changes which he has brought about have had unforeseen consequences．Who would have thought that the throwing away of a piece of Canadian waterweed would have caused half the waterways of Britain to be blocked for a decade, or that the provision of pot cacti for lonely settlers’ wives would have led to Eastern Australian being overrun with forests of Prickly Pear? Who would have prophesied that the cutting down of forests on the Adriatic coasts, or in parts of Central Africa, could have reduced the land to a semidesert, with the very soil washed away from the bare rock? Who would have thought that improved communications would have changed history by the spreading of disease-sleeping sickness into East Africa, measles into Oceania, very possibly malaria into ancient Greece?

These are spectacular examples; but examples on a smaller scale are everywhere to be found．We make a nature sanctuary for rare birds, prescribing absolute security for all species; and we may find that some common and hardy kind of bird multiplies beyond measure and ousts the rare kinds in which we were particularly interested．We see, owing to some little change brought about by civilization, the starling spread over the English country-side in hordes．We improve the yielding capacities of our cattle; and find that now they exhaust the pastures which sufficed for less exigent stock．

小题1:The following examples except ________ reflect man altering the balance of nature．

A．man is covering huge areas with new kinds of plants

B．man is facilitating the spread of animals into new regions．

C．man is killing some species on a large scale

D．man is getting to know the importance of keeping the balance of nature．小题2:What had a piece of Canadian waterweed cause?

A．Eastern Australian was overrun with forests．

B．Half the waterways of Britain blocked for a decade．

C．In parts of central Africa, the land reduced to a semidesert．

D．Disease-sleeping has been caused．小题3:What have spread diseases?

A．Disease-sleeping sickness．

B．Measles．

C．Improved communications．

D．Malaria．小题4:We make a nature sanctuary for rare birds but __________

A．some common and hardy kind of bird multiplies

B．rare kinds multiply

C．all bird multiply

D．no bird multiply小题5:The main idea of the passage is _________．

A．that man is deliberately destroying the balance of nature

B．that man has foreseen the consequences of altering the balance old nature

C．that improved communications have changed history

D．that man is altering the balance of nature

西方文化的特点之一是开放性。

贴花操作时，花纸中的气泡不会影响花面的质量.