基于另外两个列中的值，用另一个数据框中的值填充另一个数据框中的新列？(Python/Pandas)

import pandas as pd

df1 = pd.DataFrame({'A': [1, 2, 3, 4], 'B': [5, 6, 7, 8]})
df2 = pd.DataFrame({'C': [9, 10, 11, 12]})

merged_df = pd.merge(df1, df2, left_index=True, right_index=True)

merged_df['D'] = merged_df['D'].fillna(merged_df['C'])

import pandas as pd

df1 = pd.DataFrame({'A': [1, 2, 3, 4], 'B': [5, 6, 7, 8]})
df2 = pd.DataFrame({'C': [9, 10, 11, 12]})

merged_df = pd.merge(df1, df2, left_index=True, right_index=True)
merged_df['D'] = merged_df['D'].fillna(merged_df['C'])

print(merged_df)

   A  B   C   D
0  1  5   9   9
1  2  6  10  10
2  3  7  11  11
3  4  8  12  12