Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_47 (system VARCHAR, internal_hard_disk VARCHAR, s_100_slots VARCHAR, year_introduced VARCHAR)
### Question:
Which system introduced earlier than 1981 has an internal hard disk of 11 megabytes and less than 21 s-100 slots?
### Answer:
SELECT system FROM table_name_47 WHERE s_100_slots < 21 AND year_introduced < 1981 AND internal_hard_disk = "11 megabytes"