执行以下语句， 将输出？（ ）
list1=['b','c',1,2,3,4,5]
list1.append('a')
print(list1)
['b','c',1,2,3,4,5]
['b','c',1,2,3,4,5,'a']
['b','c',1,2,3,4,5,'a','a']
无输出