答案：D

考查定语从句。It was at the work shop     we did reserch to gether two years ago是省略了强调句型的句子，补充完整是It was at the work shop     we did reserch to gether two years ago that I got to know the professor over there. the work shop是先行词，在定从     we did reserch to gether two years ago中作地点状语，所以用where引导。选D。