已知列表list = ['d','r','a','g','o','n']，执行一次命令list.append('s')后，print(list)的输出结果是？（ ）
['d','r','a','g','o','n']
['s','d','r','a','g','o','n']
[ ]
['d','r','a','g','o','n','s']
