用所给词的适当形式填空

1. There are so many people in the street! It is really too ____ (crowd), isn't it?

2. At school we are all ____ (friend) to each other.

3. I've been ____ (wait) for you for one hour.

4. There is a lot of sand on the beach, so it’s _____ (sand).

5. Can you see the _____ (baby) over there? They are twins.

高温或猛烈撞击均会使化肥硝酸铵发生剧烈的分解反应，生成大量气体，放出大量的热，因而发生爆炸。已知硝酸铵爆炸反应的化学方程式为 2NH₄NO₃＝2N₂↑＋O₂↑＋4X，则X的化学式是      。