-Can I smoke here?  

-Sorry. We don't allow________here.  [ ]

A. people smoking              

B. people smoke  

C. to smoke                    

D. smoking

They had _______ computers together, so they had to overcome these _______ a network. [ ]

A. difficulty to link, difficulty to create

B. difficulty linking, difficulty creating

C. difficulties linking, difficulties creating

D. difficulty linking, difficulties to create