军事基地的地下指挥中心被标记为硬目标，因为它有厚重的钢铁防护层。

The underground command center of the military base was marked as a ___hard-target due to its thick steel protection.

这位固执的教授对新教学方法完全是个难啃的硬骨头。

The stubborn professor was a ___hard target for the new teaching methods.