_______the fight stopped,travel across the country has been quite safe.

A,意思是自从那个航班停飞了，整个城市都变得很安全了
顺治皇帝从见到她那天起就再也没有认真当官：The Emperor Shunzhi had never taken his duty seriously since he saw her that day.

说的是过去的事情，而且是过去已经完成了的事情。
when 和while的用法区别
两者的区别如下：

①when是at or during the time that, 既指时间点，也可指一段时间；

while是during the time that，只指一段时间，因此when引导的时间状语从句中的动词可以是终止性动词，也可以是延续性动词，而while从句中的动词必须是延续性动词。

②when 说明从句的动作和主句的动作可以是同时，也可以是先后发生；while 则强调主句的动作在从句动作的发生的过程中或主从句两个动作同时发生。

③由when引导的时间状语从句，主句用过去进行时，从句应用一般过去时；如果从句和主句的动作同时发生，两句都用过去进行时的时候，多用while引导，如：

a. When the teacher came in, we were talking.

当此句改变主从句的位置时，则为：

While we were talking, the teacher came in.

b. They were singing while we were dancing.

④when和while 还可作并列连词。when表示“在那时”；while表示“而，却”，表对照关系。如：

a. The children were running to move the bag of rice when they heard the sound of a motor bike.

孩子们正要跑过去搬开那袋米，这时他们听到了摩托车的声音。

b. He is strong while