以下代码输出结果是? ( )
ls =["2021","123","Python"]
ls.append([2021,"2021"])
ls.append(2021)
print(ls)
['2021', '123', 'Python', 2021]
['2020', '123', 'Python', [2021, '2021']]
['2021', '123', 'Python', [2021, '2021'], 2021]
['2020', '123', 'Python', [2021],2021]