user: Question: Which system introduced earlier than 1981 has an internal hard disk of 11 megabytes and less than 21 s-100 slots?
Columns: ['System', 'Year Introduced', 'S-100 slots', 'Internal Floppy Disk', 'Internal Hard Disk']
Types: ['text', 'real', 'real', 'text', 'text']
Sample row: ['Z-1', 1976.0, 21.0, 'n/a', 'n/a']
SQL:

assistant: SELECT "System" FROM "Table" WHERE "S-100 slots" < 21.0 AND "Year Introduced" < 1981.0 AND "Internal Hard Disk" = '11 megabytes';