运行下列程序，输出的结果是？（ ）
>>> a={1,2,3,4,5,6} >>> b={4,5,6,7,8,9} >>> print(a&b)
{1, 2, 3}
{4, 5, 6}
{7, 8, 9}
{1, 2, 3, 4, 5, 6, 7, 8, 9 }