士兵们在城门下安放了炸药包准备炸开防御。

The soldiers placed ___petards under the gate to breach the defenses.

他散布谣言想毁掉别人的名声，结果这些谣言反而成了搬起石头砸自己脚的武器。

The rumors he spread to ruin others' reputations turned out to be ___boomerangs that damaged his own credibility.