Question:
Who was the head of Sofia in Apr, 1991?

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

Answer:
Question:
Who was the head of Sofia in Apr, 1991?

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

Answer:
user
Aleksandar Karakachanov
assistant
Question:
Who was the head of Sofia in Apr, 1991?

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

Answer:
user
Todor Zhivkov
Stefan Sofiyanski
Boyko Borisov
assistant
Question:
Who was the head of Sofia in Apr, 1991?

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

Answer:
user
Aleksandar Karakachanov
assistant