军队驻扎在两国边境附近。

The army is stationed near the ___frontier between the two countries.

探险家们冒险穿越边界，进入未开发的荒野。

Explorers ventured beyond the ___frontier into the untamed wilderness.

这项研究正在拓展我们对人类大脑认知的边界。

This research is expanding the ___frontier of our understanding of the human brain.