Seasonal change means that certain objective phenomena have certain regular and obvious fluctuations with the change of seasons within a year. According to the above definition, which of the following is seasonal changes? ['Freezing weather in Changsha once every 50 years in spring of 2008.', 'From June to August 2006, the highest sales volume of watermelons in Changsha.', 'The number of postgraduates applying for examination in 2007 decreased by 72,000 compared with 2006.', "China's consumer price index rose rapidly in the winter of 2007."]