His father, a famous university professor, who is said to have started his teaching career in Australia, _____ in Nanjing University for more than twenty years.

His father, a famous university professor, who is said to have started his teaching career in Australia, _____ in Nanjing University for more than twenty years.

His father, a famous university professor, who is said to have started his teaching career in Australia, _____ in Nanjing University for more than twenty years.

1. A.
   has worked
2. B.
   worked
3. C.
   is working
4. D.
   will work

His father, a famous university professor, who is said to have started his teaching career in Australia, _________ in Nanjing University for more than twenty years.   

A．has worked      B．worked     C．is working       D．will worked