根据拼音写出相应的词语。

①牛背上牧童的短笛，这时候也成天liáo  liàng____地响着。

②看吧，由chéng  qīng____的河水慢慢往上看吧。

③我亲自抱了这雪白的大鸟回家，放在院子内。它伸长了头颈，zuǒ  gù yòu  pàn____。

④还是去年的主，还是去年的宾，他们宾主间是如何的róng róngxiè xiè____呀！

一个做匀加速直线运动的物体，初速度$v_0=2.0m/s$，它在第3 s内通过的位移是4.5m，则它的加速度为 (    )

    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.

已知a+b=3，ab=2，计算：a²b+ab²等于（　　）