an后面加什么

　　I think you have made an error in calculating the total.

　　我想你在計算總數時出了差錯。

　　We put an ad in the local paper.

　　我們在當地報紙上登了一則廣告。

　　The government's policy is an open door to disaster.

　　政府的政策為災難敞開了大門。

　　This door should only be used in an emergency.

　　這道門只能在緊急情況下使用。

　　He's an expert at getting his own way.

　　他在如何達到自己的目的方面很在行。