Which system introduced earlier than 1981 has an internal hard disk of 11 megabytes and less than 21 s-100 slots?
CREATE TABLE "cromemco_s_100_microcomputer_systems_z_8" (
  "system" text,
  "year_introduced" real,
  "s_100_slots" real,
  "internal_floppy_disk" text,
  "internal_hard_disk" text
);
SELECT "system" FROM "cromemco_s_100_microcomputer_systems_z_8" WHERE "s_100_slots"<21 AND "year_introduced"<1981 AND "internal_hard_disk"='11 megabytes';