What is the archive of the show that aired on 18april1970?
CREATE TABLE "table1_2102898_1" (
  "episode" text,
  "broadcast_date" text,
  "run_time" text,
  "viewers_in_millions" text,
  "archive" text
);
SELECT "archive" FROM "table1_2102898_1" WHERE "broadcast_date"='18April1970';