充满氢气的气球在天空中轻松地漂浮着。

The ___hydrogen-filled balloon floated effortlessly in the sky.

这家工厂使用氢作为清洁能源来发电。

This factory uses ___hydrogen as a clean energy source to generate electricity.

水分子由两个氢原子和一个氧原子组成。

A water molecule consists of two ___hydrogen atoms and one oxygen atom.