user: Question: What is the greatest Time with a Year of 1991, and Event of 4x100 m relay?
Columns: ['Year', 'Competition', 'Venue', 'Position', 'Event', 'Time']
Types: ['real', 'text', 'text', 'text', 'text', 'real']
Sample row: [1983.0, 'European Indoor Championships', 'Budapest', '1st', '60 m', 6.63]
SQL:

assistant: SELECT MAX("Time") FROM "Table" WHERE "Year" = 1991.0 AND "Event" = '4x100 m relay';