一个棱长都为a的直三棱柱的六个顶点全部在同一个球面上，则该球的表面积为(      )

    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.

如果直线a∥b，b∥c，那么直线a与c的位置关系是____