stack indicator

例句：

1. 1. The boxes are stacked neatly on the shelves.
   盒子整齐地堆放在架子上。
2. 2. The books were stacked in the corner of the room.
   书被堆放在房间的角落里。
3. 3. The luggage was stacked on the conveyor belt.
   行李被堆放在传送带上。