—Do you really know _____ at this time yesterday?　 

— Sorry, I don't know. [ ]

A. what did they do

B. what they did

C. what were they doing

D. what they were doing

（）就是在无人监督的情况下，也要坚持自己的内心真诚、光明磊落的道德信念，不做不道德的事。