Question:
Who was the head of Sofia in Dec, 2007?

Context:
 Aleksandar Karakachanov is the head of the government of Sofia from Oct, 1990 to Oct, 1991. 
 Todor Zhivkov is the head of the government of Sofia from May, 1949 to Nov, 1949. 
 Boyko Borisov is the head of the government of Sofia from Nov, 2005 to Jul, 2009. 
 Stefan Sofiyanski is the head of the government of Sofia from Nov, 1995 to Jun, 2005.

Answer:
Question:
Who was the head of Sofia in Dec, 2007?

Context:
 Aleksandar Karakachanov is the head of the government of Sofia from Oct, 1990 to Oct, 1991. 
 Todor Zhivkov is the head of the government of Sofia from May, 1949 to Nov, 1949. 
 Boyko Borisov is the head of the government of Sofia from Nov, 2005 to Jul, 2009. 
 Stefan Sofiyanski is the head of the government of Sofia from Nov, 1995 to Jun, 2005.

Answer:
user
Boyko Borisov
assistant
Question:
Who was the head of Sofia in Dec, 2007?

Context:
 Aleksandar Karakachanov is the head of the government of Sofia from Oct, 1990 to Oct, 1991. 
 Todor Zhivkov is the head of the government of Sofia from May, 1949 to Nov, 1949. 
 Boyko Borisov is the head of the government of Sofia from Nov, 2005 to Jul, 2009. 
 Stefan Sofiyanski is the head of the government of Sofia from Nov, 1995 to Jun, 2005.

Answer:
user
Todor Zhivkov
Stefan Sofiyanski
Aleksandar Karakachanov
assistant
Question:
Who was the head of Sofia in Dec, 2007?

Context:
 Aleksandar Karakachanov is the head of the government of Sofia from Oct, 1990 to Oct, 1991. 
 Todor Zhivkov is the head of the government of Sofia from May, 1949 to Nov, 1949. 
 Boyko Borisov is the head of the government of Sofia from Nov, 2005 to Jul, 2009. 
 Stefan Sofiyanski is the head of the government of Sofia from Nov, 1995 to Jun, 2005.

Answer:
user
Boyko Borisov
assistant