根据句子意思和所给的首字母写出所缺单词。

1. Oxygen (氧气) is a kind of u_______ gas.

2. The Yellow River is the s________ longest river in China.

3. I think it u________ to see the ghosts (鬼). You must be joking.

4. There are many a_______ who want to fly to the moon.

5. I felt surprised and scared when I saw the t________ monsters.