教育是社会发展的基石。

Education is the ___keystone of social development.

这座拱门的稳定性完全依赖于顶部的拱心石。

The stability of the arch depends entirely on its ___keystone at the top.

古罗马桥梁的拱顶中央都有一块重要的拱心石。

Every Roman bridge has a crucial ___keystone at the center of its arch.