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