根据以下内容，回答下列各题。

Man’s first real invention, and one of the most important inventions in history, was the wheel．All transportation and every machine in the world depend on it．The wheel is the simplest yet perhaps the most remarkable of all inventions, because there are no wheels in nature-no living thing was ever created with wheels．How, then, did man come to invent the wheel? Perhaps some early hunters found that they could roll the carcass of a heavy animal through the forest on logs more easily than they could carry it．However, the logs themselves weighed a lot．

It must have taken a great prehistoric thinker to imagine two thin slices of log connected, at their centers by a string stick．This would roll along just as the logs did, yet be much lighter and easier to handle．Thus the wheel and axle came into being and with them the first carts．

The wheel is important because _______．

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根据以下内容，回答下列各题。

The death of a child, _51_ one’s own child,is something the mind can hardly comprehend．It seems togo _52_ nature．The end of a life that is still forming_53_ has just begun is an almost unthinkable cruelty．

But it happens．Parents sometimes must witness the illness and death of the children_54_ whom they gave birth．And _55_ will ever fill theempty place that the dead child leaves behind．Parents_56_ to die before their children, which is the natural order of life events．When the situation is reversed and a child dies _57_ the parents itseems wrong．

Yet we never know why one person dies andanother lives．But we do know _58_ a child is gone, theparents still have a future, as grey and worthless as it may at first seem．Death may take the child, but the love never _59_．

The sorrow after a child’s death is one ofthe longest and hardest type of _60_ to live with．And ittakes long to deal with the parents’ own emotions．

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