侦察小队在敌后防线活动时被发现了。

The reconnaissance team was discovered while operating behind enemy ___lines.

勇敢的士兵们坚守前线，抵御了多次攻击。

Brave soldiers held the front ___line against repeated assaults.

将军命令他的部队加固防御战线。

The general ordered his troops to fortify the defensive ___line.