社会结构的研究是对社会关系系统的研究。

He tried to weigh his soul to see if it was a poet's soul.

Positive emotions broaden our awareness and encourage novel, varied, and exploratory thoughts and actions. Over time, this broadened behavioral repertoire builds skills and resources.

什么是对立的结合，也是对立的分离。

植物杂交实验需要最仔细的关注和极大的耐心。