字母锁

例句：

1. 1. The letter 'X' is the key to solving this puzzle.
   这个谜题的关键在于字母'X'。
2. 2. The safe was locked and we couldn't get in.
   保险箱被锁上了，我们进不去。
3. 3. She found the letter she had been looking for under the lock.
   她在锁下找到了她一直在找的信。