The semi-finals have been played exclusively at the rebuilt Wembley Stadium since 2008, one year after it opened and after it had already hosted a final (in 2007). For the first decade of the competition, the Kennington Oval was used as the semi-final venue. In the period between this first decade and the reopening of Wembley, semi-finals were played at high-capacity neutral venues around England; usually the home grounds of teams not involved in that semi-final, chosen to be roughly equidistant between the two teams for fairness of travel. The top three most used venues in this period were Villa Park in Birmingham (55 times), Hillsborough in Sheffield (34 times) and Old Trafford in Manchester (23 times). The original Wembley Stadium was also used seven times for semi-final, between 1991 and 2000 (the last held there), but not always for fixtures featuring London teams. In 2005, both were held at the Millennium Stadium.

Answer this question, if possible (if impossible, reply "unanswerable"): What was the top most used venue?
Villa Park in Birmingham (55 times)