在《最后一课》中，下课后，韩麦尔先生“仍站在那里，头靠着墙壁，不说话”，而本文中的瑞宣老师却“狠了狠心，他走下了讲台”，“他迈门坎，没迈利落，几乎绊了一跤”，韩麦尔先生和祁瑞宣老师的这种行动说明了什么？

下列各句中，没有语病的一项是（   ）

运用所学的化学知识回答：

地球自转产生的现象是（    ）

    It seems to be strange to you there is a blind spot(点) on the eyes. Here is an interesting experiment that can make something disappear, when one eye is open.
　Make a card about the size of a postcard and write two English letters L and R on it, L on the left and R on the right. First, hold the card about 80 cm away and you see both the letters. Then close your right eye and look at the letter R only with your left eye. And now, as you move the card slowly towards you, you'll find the letter L disappearing. But if you move the card nearer to your face, the letter will be seen again. Now do the same experiment with your left eye closed, you'll find the letter R disappearing.
    Why does the letter disappear? It is because there is a blind spot on the eye. When the image of the letter falls on the blind spot, it won't be seen. That is why either of the letters disappears.