她离开派对的原因是她累了。

The ___reason she left the party is that she was tired.

你能告诉我你改变主意的原因吗？

Can you tell me the ___reason you changed your mind?

我找不到我的铅笔了，这就是我需要一支新铅笔的原因。

I can't find my pencil, and that's the ___reason I need a new one.