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


4.1     A:      Zuótiān xiàwǔ nǎr le?  


                   Where did you go yesterday afternoon?


          B:      túshūguǎn le.  


                   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.  


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


4.2     A:      Shàng zhōumò tāmén dào nǎr le?  


                   Where did they go last weekend?


          B:      Tāmén dào Shànghǎi le.  


                   They went to Shanghai.