Poland joined the European Union in 2004 as part of a major "big bang" enlargement that admitted how many countries simultaneously?

On 1 May 2004, the European Union undertook its largest enlargement to date, admitting ten new member states simultaneously: Poland, the Czech Republic, Slovakia, Hungary, Slovenia, Estonia, Latvia, Lithuania, Cyprus and Malta, expanding the bloc from fifteen to twenty-five members in a single round widely termed the 'big bang' enlargement. Poland, as the largest of the ten new entrants both by population and territory, played a particularly prominent role in this round and has since become one of the EU's major economies. The option 5 understates the scale considerably and might be mistaken for a smaller, earlier or later enlargement round, such as the 2007 accession of Romania and Bulgaria, which admitted only two countries. The option 8 similarly understates the true figure and could be confused with Austria's count of bordering countries or other unrelated statistics entirely. The option 12, meanwhile, overstates the 2004 round, though it is close to the total if one mistakenly includes the two 2007 entrants or confuses it with the EU's aggregate membership figure at some later point. A common exam confusion is blending the 2004 and 2007 enlargement rounds together, since both involved former Eastern Bloc states joining in relatively close succession; keeping them numerically and chronologically separate, 10 in 2004 versus 2 in 2007, Bulgaria and Romania, is essential. The key revision fact: Poland joined in the historic 2004 'big bang' round of exactly ten countries.