user: ### NOW ANSWER:
Question: What is the Lap Length of the race with a Grand Prix FIA Category?
Columns: ['Date', 'Race', 'Race track', 'Category', 'Lap length']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['July 21, 1946', '1st Grand Prix des Nations', 'Circuit des Nations', 'Grand Prix FIA', 'km (mi)Switzerland']
SQL:

assistant: SELECT "Lap length" FROM "Table" WHERE "Category" = 'Grand Prix FIA';