Every object has weight. But the center of its weight is not always in the middle of the object. If the top of an object is heavier than its bottom, it will fall over easily. We say that its center of gravity is high. But if the bottom is heavier, it won't fall easily. Then we say that its center of gravity is low.
    Look at the first picture. This empty box must fall over, because its center of gravity (C) is not above the line of its bottom (AB).
Now look at the second picture. There is some heavy iron in the bottom of this box. It will not fall, because its center of gravity is above its bottom.

阅读文段，回答问题。

使至塞上   

    王维

                                                           单车欲问边，属国过居延。征蓬出汉塞，归雁入胡天。

                                                           大漠孤烟直，长河落日圆。萧关逢候骑，都户在燕然。

卖花声

张舜民

楼上久踟躇。地远身孤。拟将憔悴吊三闾。自是长安日下影，流落江湖。

烂醉且消除。不醉何如。又看暝色满平芜。试问寒沙新到雁，应有来书。

【注】张舜民，北宋文学家、画家。元丰四年从高遵裕征西夏，无功而返。次年十月遭劾奏，谪监郴州。南行途经岳阳作此词。