—I'm going to the post office.
—_______ you're there, can you get me some stamps?

A. As    B. While  C. Because  D. If

答案解析：B为正确答案

以上为连词while引导时间状语从句意思“当…的时候，在…期间 ”

参考译文：

-I'm going to the post office.

我打算要去一趟邮局。

-While you are there, can you get me some stamps?

当你在那里的时候，你能帮我弄一些邮票吗？

(1)while  引导让步状语从句 意思“虽然，尽管”

In a word, while the prospects are bright, the road has twists and turns.
总之,虽然前途是光明的,但是道路是曲折的。

While I admit his good point,I can see his shortcomings.  

虽然我承认他的优点,我还是能看到他的缺点。

While the numbers of such developments are relatively small, the potential market is large. 

尽管这种增长的数量相对较小，但是潜在的市场是巨大的。

While the modelling business is by no means easy to get into, the good model, male or female, will always be in demand. 

尽管模特行业绝不是那么容易进入的，但是好的模特，不论男女，总是需要的

(2)while 并列连词，表示2种情况的对比，意思"然而"

Motion is absolute while stagnation is relative.   
运动是绝对的,而静止是相对的。

(3)引导时间状语从句，在...期间，当...时候，（从句谓语动词为延续动词或者状态动词）

You don't want to marry while you are studying.  

在学习期间,你不要结婚。

While we were talking,he came in.  

我们正在谈话时,他进来了。

While in prison, he wrote his first novel.  

他在狱中写出了第一部小说。