Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
Which system introduced earlier than 1981 has an internal hard disk of 11 megabytes and less than 21 s-100 slots?

### Inputs:
table_name: table, columns: ['System', 'Year Introduced', 'S-100 slots', 'Internal Floppy Disk', 'Internal Hard Disk']

### Response:
SELECT System FROM table WHERE S-100 slots < 21 AND Year Introduced < 1981 AND Internal Hard Disk = 11 megabytes