Start by naming the Top 10 (incl ties) HR leaders for 1970. For 1971, name the Top 10 assuming the 1970 Top 10 had Retired. For 1972, assume those in the 1970 & 1971 lists had retired, and so on.
Basically - for a given year, for a player to be in that list they must be among the top 10 HR hitters for that year NOT INCLUDING those who would've been listed for a previous year in the quiz. So for 1972, you are looking for players who weren't in the 1970 Top 10, nor those who were in the 1971 Top 10 (a Top 10 which excluded the 1970 Top 10)...and so on. So no player will be listed more than once.