CREATE TABLE table_204_70 (id DECIMAL, "pick #" DECIMAL, "nfl team" TEXT, "player" TEXT, "position" TEXT, "college" TEXT)

what is the average number of defensive backs chosen in the seventh round ?
SELECT COUNT(*) FROM table_204_70 WHERE "position" = 'defensive back'
SELECT COUNT(*) FROM "table_204_70" WHERE "position" = 'defensive back'