单选题 All the documents must be kept confidential, so all employees are advised to check that all doors are locked properly ______ leaving the office.
【正确答案】 A
【答案解析】[解析] 词汇：介词
1．这是一道词汇题，选项均为介词。
2．原文的意思是“离开办公室前要关好门”，因此A．before为正确选项。