我不是因为快乐而歌唱，而是因为歌唱而快乐。
I don't sing because I'm happy; I'm happy because I sing.
社会结构因此被定义为实际存在的社会关系的总网络。
Sustainability is no longer an option; it's a necessity for any business that wants to thrive in the 21st century.
Natural experiments are a powerful tool for understanding causal relationships in the real world.
"The world is full of victims, and I am one of them."
译文：世间总有得不到的东西，也总有办不到的事。