frontier|frontiers

他们在边境上部署了卫兵。

They placed guards on the ___frontier.

他拿了它并向德国边境逃去。

He took it and escaped towards the German ___frontier.

两国协商了一条新的边境线。

The two nations negotiated a new ___frontier.

研究人员正在不断拓展科学认知的前沿。

Researchers are constantly pushing the ___frontiers of scientific understanding.

她的作品位于当代艺术表达的前沿。

Her work is at the ___frontiers of contemporary artistic expression.

这一发现拓展了医学可能性的边界。

This discovery has expanded the ___frontiers of what is medically possible.