fall opening

例句：

1. 1. The door's fall opening is a bit tight.
   门的开合有点紧。
2. 2. The engineer closed the fall opening to prevent leakage.
   工程师关闭了开合以防止泄漏。
3. 3. The scientist discovered a new mechanism that could open and close the fall opening with a simple switch.
   科学家发现了一种新的机制，可以通过简单的开关来打开和关闭开合。