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.

（）是指公司和员工约定在将来某一时期内以特定价格购买一定数量的公司股权，购股价一般参照股票的现行价格确定。

A.现股计划

B.期股计划

C.期权计划

D.干股计划