两个物体之间的作用总是相互的．物体间相互作用的这一对力，常常叫做作用力和反作用力．作用力和反作用力之间存在什么样的关系呢？小高进行了如下的探究：

把甲、乙两个弹簧测力如图所示的方法连接在一起，手拉弹簧测力计甲，可以看到两弹簧测力计的指针同时移动．弹簧测力计甲的示数表明弹簧测力计乙对它的拉力，而弹簧测力计乙的示数表明弹簧测力计甲对它的拉力．改变手拉弹簧测力计力的大小，两弹簧测力计的示数也随着改变，而且两弹簧测力计的示数总是相等．请回答：

从《白桦林的低语》中的“____”这句话,我感受到了看林人对工作的兢兢业业、恪尽职守;从“____”这句话,我感觉林业工人能够苦中作乐,从枯燥的生活中发现诗意。

    History is the story of us, human beings. Everyone who has lived and everything that has h____ are part of history. But history is not only what happened in the past. The word also means the study of the past, through old w____records and other kinds of facts.

    Why do we study history? One r____is that we enjoy it. A good story is even better if it is true, and history is full of good s____though they can be terrible too. A more serious reason is to understand o____and the world we live in. To understand events happening now, we need to know their causes. We cannot understand the p____if we know nothing about the past. History helps us understand why things are the way they are.

    Of course, history books cannot tell us everything about the past, not even everything that i____recorded. A history of the world has to pick out the people and events t____changed the world.

    Since ancient times, great civilizations have appeared in d____countries and regions. Usually, only three or four existed at the same time. While they lasted they changed the l____not only of their own people, but of their neighbors, today.

血液流经下列哪一器官后，由静脉血变成了动脉血（　　）