What is the pick number for position of D from the University of Maine (NCAA)?
CREATE TABLE "1988_nhl_entry_draft" (
  "pick_num" real,
  "player" text,
  "position" text,
  "nationality" text,
  "nhl_team" text,
  "college_junior_club_team" text
);
SELECT "pick_num" FROM "1988_nhl_entry_draft" WHERE "position"='d' AND "college_junior_club_team"='university of maine (ncaa)';