user: Question: What is the lowest weight class (kg) that has sofia, bulgaria as the venue?
Columns: ['Year', 'Event', 'Venue', 'Place', 'Weight class (kg)']
Types: ['real', 'text', 'text', 'text', 'real']
Sample row: [1963.0, 'FILA World Championships', 'Helsingborg , Sweden', '4th', 97.0]
SQL:

assistant: SELECT MIN("Weight class (kg)") FROM "Table" WHERE "Venue" = 'sofia, bulgaria';