已知dataframe df如下 :data1 data2 key1 key20 1 6 a one 1 2 7 a two2 3 8 b one 3 4 9 b two 4 5 10 a one 以下写法错误的是____?
A. df.groupby('key1')['data1'].mean()
B. df.groupby('key1')[['data2']].mean()
C. df.groupby(['key1', 'key2'])[ 'data1','data2'].mean()
D. df.groupby(['key1', 'key2']) ['data1'] ['data2'].mean()