correction for sag

例句：

1. 1. The fabric sags when it's wet.
   当布料湿的时候会下垂。
2. 2. The car sags when the engine is off.
   汽车引擎熄火时会下沉。
3. 3. The weight of the bag sags as it fills with water.
   随着袋子里装满水，重量下沉。