CREATE TABLE table_31673 ("Year" TEXT, "Broadcast date" TEXT, "BBC One total viewing" TEXT, "BBC One Rank" TEXT, "BBC Two total viewing" TEXT, "BBC Two Rank" TEXT)

In what year was BBC 2 13th in rank?
SELECT "Year" FROM table_31673 WHERE "BBC Two Rank" = '13th'
SELECT "Year" FROM "table_31673" WHERE "BBC Two Rank" = '13th'