备用石

例句：

1. 1. We need to havespare stones ready in case ofemergencies.
   我们需要准备备用石头以防紧急情况。
2. 2. The carpenter useda piece of spare stone to repair thedoor.
   木匠用一块备用石头来修理门。
3. 3. The builders had to use sparestones from their stock to complete the projecton time.
   建筑工人不得不使用库存中的备用石头按时完成项目。