D. Le is used to ask or answer questions about where someone went/has gone.

 

4.1     A:      Zuótiān xiàwǔ nǎr le?  

                   昨天下午你去哪儿了?

                   Where did you go yesterday afternoon?

 

          B:      túshūguǎn le.  

                   我去图书馆了。

                   I went to the library.

Note that the following sentence answers the question, “what did you do?” So it is not exactly the answer for the question above, “where did you go?”

 

-le tàng túshūguǎn.  

我去了一趟图书馆。

I went to the library. (Lit. I took a trip to the library.)

 

4.2     A:      Shàng zhōumò tāmén dào nǎr le?  

                   上个周末他们到哪儿去了?

                   Where did they go last weekend?

 

          B:      Tāmén dào Shànghǎi le.  

                   他们到上海去了。

                   They went to Shanghai.