有如下程序段，程序运行后输出的结果是？（ ）
set1 = {1,2,3, 4} set2 = {3, 4,5, 6} difference = set1 - set2 print(difference)
{1,2,3,4,3,4,5,6}
{1,2,3,4,5,6}
{1,2}
程序出现错误