①蹦蹦跳跳（AABB式的词语）：____、____

②形容高兴的四字词语：兴高采烈、____、____

③含有数字的四字词语：四面八方、____、____

若关于的一元二次方程$k{x}^2-6x+9=0$有两个不相等的实数根，则k的取值范围是____ 

    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.

当你到商店购买食品时，应该怎么做？