孩子们在泳池里上下漂浮，玩得很开心。

The kids were happily ___bobbing up and down in the pool.

小船在波涛中上下颠簸。

The small boat was ___bobbing up and down in the waves.

小女孩用快速的点头动作向老师问好。

The little girl greeted her teacher with a quick ___bobbing motion.