PRAGUE — A quiz for history buffs. Twenty years ago — on June 4, 1989 — three events shaped a fateful year. Which do you remember most vividly, and which most changed the world?: (a) the bloody denouement of the protests on Tiananmen Square; (b) the death of Iran's revolutionary cleric, Ayatollah Ruholla Khomeini; and (c) the Polish elections.