at command的用法和樣例：
例句
- She has a large English vocabulary at her command.
她能靈活運用掌握的大量英語辭匯。 - The sense of this word is hard to come at.
這個詞的意思不容易掌握。 - Here Mr. Casey had placed at my disposal the agreeable villa he was using.
這里，凱西先生把他自己居住的那座舒適的別墅供我自由使用。 - Users can make their public keys freely available or place them at a key distribution centre for others to access.
用戶可以讓公開密鑰自由使用或者把公開密鑰放在可由別人存取的密鑰分配中心。
