下列海洋的空间利用方式属于对海底空间利用的是(　　)

    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.

①人民是国家和社会的主人

②公民依法享有监督权

③公民依法参与政治生活

④国家的一切权力属于人民

在一个减法算式中，被减数、减数、差三个数的和是168，减数与差的比是2：1，减数是____．